[Dune] Questions concerning Raviart-Thomas elements

[Andreas Dedner]

These are ``arbitrary`` order RT element for (as stated) nd-simplex 
elements and are based on a different construction principle using the 
structure of the reference elements. We know how to extend them to cube 
elements (and prism) but have not yet got around to doing it.
So they are unrelated to raviartthomas0??d.hh...

They are tested in testgenericfem.cc.

Greetings
Andreas

> - Why are there the files
> dune/localfunctions/raviartthomas.hh
> dune/localfunctions/raviartthomas/raviartthomasbasis.hh
> dune/localfunctions/raviartthomas/raviartthomasprebasis.hh
> dune/localfunctions/raviartthomas/interpolation.hh
> present? They seem to be unrelated to the actual implementations of
> the RT elements. And I could not find an use beside test.cc in the
> same directory.