[Core] Making Variable and VariableData constexpr

[sunethwarna]

📝 Description
As in the title, this PR tries to make the Variable and VariableData constexpr. One advantage is that we can now use switch jump table optimizations with variables.

This makes Variable::mZero static, which, anyway, was always default-initialized. No custom zero definitions were found anywhere in Kratos [ Check #14253 ].

Closes #14253

Eg:

Kratos/kratos/tests/cpp_tests/containers/test_variables.cpp

Lines 95 to 101 in 4eff549

	const auto& func_variable = [](const Variable<double>& rVariable) -> int {
	switch (rVariable.Key()) {
	case PRESSURE.Key(): return 1;
	case DENSITY.Key(): return 2;
	default: return 3;
	}
	};

or

Kratos/kratos/tests/cpp_tests/containers/test_variables.cpp

Lines 107 to 113 in 4eff549

	const auto& func_variable_data = [](const VariableData& rVariableData) -> int {
	switch (rVariableData.Key()) {
	case VELOCITY.Key(): return 1;
	case ACCELERATION.Key(): return 2;
	default: return 3;
	}
	};

Lets see whether what problems the CI gives.

🆕 Changelog

• Made all the uses of Variable and VariableData to be const correct.
• Made Variable::mZero static and default initialized to zero.
• Made the Variable and VariableData constexpr
• Added a unit test with the switch

[roigcarlo]

Took a first look and unless I missed something this looks fine. I focused on checking that moving from external to static inline kept the functionality unchanged. This also improves the safety of the resulting code as it delegates the responsibility of merging the definitions of the variable to the compiler, and not to the runtime.

gj @sunethwarna !!

[loumalouomega]

LGTM

[avdg81]

On behalf of @KratosMultiphysics/geomechanics, we're okay with the changes proposed for the GeoMechanicsApplication 👍