Retrait de compute_elastic des choix matériaux et création de MaterialEval

MaterialEval will give evaluators and their types for stress, pre-stress, tangent matrix, elastic matrix, Softening function, ... Compute_elastic will now have only one template parameter : EvalMaterial

Template arguments will nom be in MaterialEval and no longer in compute_elastic

How it is currently done for the softening function For now, only constant softening and CZM equivalent softening functions are available. How it should be done: Add user defined softening function. Two possibilities: • The durable way: if the new softening function is supposed to be kept and reuse by other users, add it into the sources as it was done for the CZM equivalence • To do some tests: define a softening function in the main.cc/h , give it to the code, implement a xEval which uses this function