Introduce isotropic hardening
- a class Nonlinearity is added which implements value(), gradient(), hessian() and subdifferential() for a given DissipationDensity
- the class DissipationFunctional is completely cleaned up and now better to read
- the coefficient vectors are given in std::vector format now
- we need the elementVolumes and the previousEta for evaluation of the nonlinearity and the direct solver
Edited by Jaap, Patrick