Add utility function writeBasis()

This plots writes all basis functions as vtk sequence
for viewing them with paraview. This is _very_ helpful
when investigating/debugging a dune-functions basis
and affine constraints e.g. for hanging nodes, Dirichlet
BC, ... .

Ideally this should be part of dune-functions, but the
latter does not depend on dune-vtk and furthermore does
not know about constraints so far.