Create a sasmodels.lib support package for pure python models

[pkienzle]

Make a common namespace for the support libraries for pure python modules.

Currently they are in sasmodels.special and the sasmodels.TwoYukawa package. There may also be a sasmodels.quadrature module if PR #699 is merged.

Note: PEP8 says package names should be snake_case. Somehow both the author (me) and the reviewers messed up with TwoYukawa. We can fix that if we move support libraries to lib.

Originally posted by @pkienzle in #699 (comment)

[marimperorclerc]

OK, sure, about the quadrature module, we can move it elsewhere on the branch for adding nanoprisms.

Just, there is already a subfolder called lib here:
C:\SasView-6.1.2\sasmodels\models\lib

... might be misleading to create another lib folder in this other location:
C:\SasView-6.1.2\sasmodels\lib

Better to chose another namespace related to the pure python models ?
Like libpy instead of lib ??? or something else ?