Add explicit template instantiation of class FoamGrid

Summary

This MR fixes some problems with explicit template instantiation of the class FoamGrid. And a library including these explicit instantiations is added.

Details

The problems come from explicit specializations of methods for some entity dimensions. If these methods are instantiated for all grid dimensions it crashes. Instead of this explicit overloads, I have added templated member functions that are instantiated only for the correct dimensions.

Some additional commits in the MR:

• Fix missing forward declaration of types used in friend declarations
• Rename foamgrid.cc to foamgrid.impl.hh since it was not a source file, but a header file of template implementations. Instead a new file foamgrid.cc is added, containing just the template instantiations.
• Move the include of foamgrid.impl.hh out of the Dune namespace and add include guards.

mentioned in issue #16 (closed)

@tkoch commented on a deleted MR:

@simon.praetorius Thanks! With the explicit template instantiations will I still be able to use FoamGrid<1, 10> (I don't know if that ever worked or was used but it was supposed to) or does that not work anymore? I always wondered about that aspect of the feature and never got around trying it out.

Yes. Only those symbols explicitly instantiated are put into the library. But, everything else is instantiated on request, e.g. when you write FoamGrid<1, 10> in your executable.

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• 3982001c - [lib] Add foamgrid library as module library

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@simon.praetorius I get a linker error with your version (something related to Dune::Exception) which disappears after my last commit. Is there something wrong with this approach?