Add support for material classes

[hverhelst]

We introduce a family of classes derived from gsMaterialBase that can be used as a black-box for constitutive laws in the gsElasticityAssembler.

The material classes implement functions like eval_stress_into and eval_matrix_into which basically return the stress and material tensors.

They can be evaluated on points (using pre-computed deformation gradients and strains)

gsMaterialBase<T> * material;
gsMaterialData<T> data;
// ...
material->precompute(&undeformedPatch,&deformedPatch,patch,u,data);
gsMatrix<T> S, C;
material->eval_stress_into(data,S);
material->eval_matrix_into(data,C);

Or via an evaluator (gsMaterialEval) is provided, which behaves as a gsFunction;

gsMaterialEval<T,gsMaterialOutput::S> evaluator(material,undeformedPatch,deformedPatch);
evaluator.piece(patch).eval_into(u,S);