Feature: Tresca plasticity and nonlinear isotropic hardening

This MR includes

• Implementation of von Mises / Tresca dissipation in one dissipation functional with runtime switch
• Assembling data structures in dune-functions style assemblers (isotropic-hardening.cc)
• Dropping dune-pdelab (primalplasticity-p1p0.cc)
• Encapsulation of all parameters into a class
• Various hardening rules: linear kinematic, (non-)linear isotropic with runtime parameters
• 2D/3D models for von Mises
• 3D model for Tresca (2D Tresca is equivalent to 2D von Mises)