Refactoring of finite element classes

[SerafeimBakalakos]

Right now it is extremely difficult to reuse the code in these classes for other problem types, boundary conditions, etc.

[SerafeimBakalakos]

• I have decoupled the integration rules and interpolations (shape functions) from the finite elements. They can be used by the finite element classes or others.
• All continuum finite elements are now represented by the same class, with different interpolations and integration rules. This decreases code duplication.
• The above have been fully implemented and tested for Quad4 and Tri3 elements, as of yet.
• Please take a glance at the new classes in ISAAR.MSolve.FEM.Integration, ISAAR.MSolve.FEM.Interpolation and ISAAR.MSolve.FEM.Elements and comment on how they can be improved (other than the missing non-linear and embedding functionality).

[dimtsap]

@SerafeimBakalakos I have almost completed the 3D case. Please note that some caching was missing so I corrected this in both 2D and 3D case.