[dune-pdelab] Why are constraints in finiteelementmap/

[Christoph Grüninger]

Hi PDELab,
I move the constraints around, the result is in
  p/gruenich/fixes

I think the interiornodeconstraints.hh is an implementation,
so I left it in constraints/.

In a separate commit I renamed the constraints implementation
headers. They are already in constraints/, now they don't
have this word in their file name.

If you like the changes and merge them into master, I'll
modify dune-pdelab-howto accordingly.

Have a nice day,
Christoph

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