[testpoisson] Adapt to new QkFEM constructor with GridView parameter
[PkFEM] Unify and clean up Pk finite element maps

- Deprecate redundant dimension parameter in PkLocalFiniteElementMap -
  we can just extract it from the GridView.

- Use PkLocalFiniteElement across all dimensions, which required some
  changes to the way the finite element is looked up in 2D.

- Move all implementations to a new common base class
  fem::PkLocalFiniteElementMapBase, which is specialized on the
  dimension.

- All implementations are now in pkfem.hh.

- All older, specialized finite element maps are now deprecated along
  with their header files.
[testpk] Adapt to new PkLocalFiniteElementMap interface