[FiniteElement] add missing size and partial methods

The dune-localfunctions interface for the finite element and for the local basis contains some functions that were not yet implemented for the finite elements provided by PDELab.

The local basis virtual interface contains the partial function and the finite element virtual interface contains the size function.

We add these functions to the finite elements in PDELab. The partial method is only implemented for order 0 (forwarding to evaluateFunction) and throws a Dune::NotImplemented otherwise. This follows some implementation in dune-localfunctions. The size method forwards to basis.size().