Implement 'setup' for Pk element transfer operators
requested to merge patrick.jaap_at_tu-dresden.de/dune-solvers:feature/P0-element-tranferoperators into master
'setup' is now capable of using Lagrangian basis functions of any order k >= 0. For easy access of the global basis size, a dependency of dune-functions is added. For back-compatibility, the constant k=1 is set as default value.