[NL] Add Eigen fixed and dynamic ShapeMatrixPolicy.
[ML] Make elements' dimension a constexpr.

Needed for choosing integration method at compile time.
[NL] Add a GaussIntegrationPolicy.

Chooses regular gauss integration but for triangles.
[NL] Remove integration method from isoparam. FE.
[NL] Set integration method in TestFe explicitly.

For this test GaussIntegrationPolicy is used.
[ML] Add compile-time pow() for int exponents.
[NL] Add MeshElement type to ShapeFunctions.
[NL] Remove  T_MESH_ELEMENT from isoparam. FE.

This type is present in the shape function classes.
[T] Remove extra FeType struct using template alias.
[T] Test eigen dynamic matrices only if eigen available.
[T] Remove DefaultMatrixType. Add template to dyn. matrices.
[T] Rename T_MATRIX_TYPES to ShapeMatrixPolicy.
[T] Rename T_FE_TYPE and T_FE to TestFeType.
[T] Make TestCase's ShapeMatrixPolicy a template-template.

Then the redundant argument can be removed.
[T] Add ShapeFunction type to TestFe* types.
[T] Use NumLib::ShapeMatrixPolicy instead of own type.
[NL] Use ShapeMatrixPolicy for isoparam. FE.
[T] Forward ShapeMatrixPolicy to TestFe types.