Add native OBJ, STL, PLY, SU2 and Nastran mesh I/O

[vijaiaeroastro]

This PR makes progress on #44 by adding direct support for five commonly used
mesh-exchange formats through Mmg's existing public APIs and generic file
dispatch. The readers and writers are implemented directly in Mmg, without
requiring an additional runtime, and are available consistently to the C, C++,
Fortran and command-line interfaces.

I could not use meshio directly for this implementation because it is a
Python/NumPy library, while these readers and writers need to be available from
Mmg's native C, C++ and Fortran APIs without requiring a Python runtime.
meshio still provided a useful interoperability reference for the formats it
supports.

I intend this as a substantial first step toward #44 rather than closing the
issue. I have left formats such as Gmsh 4 and CGNS for follow-up PRs.

What is included

• OBJ for MMGS

  • Reads polygonal faces and triangulates them for MMGS.
  • Supports positive and negative OBJ indices.
  • Maps groups and materials to stable Mmg references. Explicit
    mmg_ref_<id> names preserve the requested reference.
  • Writes a companion MTL file and uses materials to preserve triangle
    references across a round trip.

• STL for MMGS

  • Reads both ASCII and binary STL, with content-based input detection.
  • Writes binary STL using the standard .stl extension.
  • Welds repeated facet vertices with a scale-aware tolerance, including for
    meshes with very small coordinates.

• PLY for MMGS

  • Reads ASCII and binary little-endian PLY.
  • Handles arbitrary supported property ordering and ignores unrelated
    properties.
  • Writes binary PLY by default and ASCII PLY with the .plya extension.
  • Emits standard PLY 32-bit integer types. The reader remains permissive
    toward non-standard 64-bit integer properties, while the writer reports an
    error if a value cannot be represented by the PLY 1.0 scalar types.

• SU2 for MMG2D, MMG3D and MMGS

  • Reads and writes native 2D, 3D volume and 3D surface SU2 meshes.
  • Supports triangles and quadrilaterals in 2D, and tetrahedra and prisms in
    3D, together with boundary markers.
  • Imports hexahedra and pyramids by conformingly decomposing hybrid volume
    meshes into tetrahedra. A warning is emitted when this conversion occurs.
  • Preserves explicit mmg_ref_<id> marker references and assigns
    collision-free fallback references to ordinary marker names.
  • MMGS imports triangular and quadrilateral faces from 3D marker sections,
    splitting quadrilaterals deterministically. It also accepts standalone
    surface-only files that place those faces in NELEM, provided no volume
    cells are mixed in.
  • MMGS output uses reference-grouped 3D marker sections, matching the usual
    SU2 representation of a boundary surface.

• Nastran bulk data for MMGS and MMG3D

  • Reads common free-field and fixed-field mesh cards and writes a portable
    bulk-data representation.
  • Supports surface triangles/quadrilaterals and volume tetrahedra/prisms.
  • Imports CHEXA and CPYRAM cells through a conforming tetrahedral
    decomposition, with a warning when conversion is required.
  • Preserves Mmg references through property IDs and $MMG_REF metadata.

The new formats participate in the existing extension-based generic I/O path;
the established readers are otherwise left intact.

API and compatibility

• Adds format identifiers after MMG5_FMT_Unknown without changing its public
  numeric value.
• Adds format-specific load/save entry points to the relevant MMG2D, MMG3D and
  MMGS public APIs.
• Adds corresponding Fortran wrappers and generated constants.
• Keeps format behavior local to each Mmg variant: surface-only formats are
  not exposed as volume readers, and SU2/Nastran support follows the element
  types represented by the relevant API.
• Introduces no new external runtime dependency.

Testing

Each format has API-level tests using actual mesh files generated by the test
programs. The tests cover generic and format-specific entry points, round trips,
reference preservation, element ordering, malformed or unsupported input, and
the hybrid-cell conversions used by SU2 and Nastran.

Additional checks include:

• ASCII/binary STL detection and nanoscale vertex welding.
• PLY property reordering, CRLF input, large floating-point coordinates,
  standard signed/unsigned reference output and 64-bit input policy.
• OBJ polygon triangulation, negative indices, group/material precedence and
  MTL output.
• SU2 and Nastran shared-face conformity, orientation, volume preservation and
  reference mapping.
• SU2 surface extraction from volume-grid markers, standalone surface input,
  quadrilateral splitting and MMGS round trips.
• Builds and focused tests with both 32-bit and 64-bit MMG5_int.
• AddressSanitizer runs for the new readers and writers.
• Shared-library and Fortran builds matching the failing GitHub Actions
  configuration.
• PLY output interoperability checked with hapPLY.

Follow-up work

I intentionally kept the following formats out of this PR so that their design,
dependencies and review remain manageable:

• Gmsh 4.x (especially 4.1): add native surface and volume support for the
  block-based $Entities, $Nodes and $Elements model. Version detection
  should select the new path without competing with or regressing the existing
  legacy Gmsh reader. Physical groups, entity tags, sparse node tags and both
  ASCII and binary encodings need explicit tests.

• CGNS: add surface and volume I/O through optional libcgns/HDF5 support.
  This should use the project's established optional-dependency conventions,
  with a clear CMake feature switch and useful behavior when CGNS is not
  installed. Element sections, boundary conditions, families and Mmg
  references need a documented mapping.

• Broader interoperability fixtures: add a small curated corpus produced by
  Gmsh, SU2, common CAD/meshing tools and Nastran-compatible solvers as the
  native readers mature.

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