Feature/gridformat writer

What this MR does / why does DuMux need it:

Adds gridformat as git submodule and a wrapper around its generic readers/writers in order to provide I/O from/to various grid file formats.

TODO:

• Discuss class and file names. The current choice was made s.t. the include is #include <dumux/io/gridwriter.hh>, since it's generic over the formats...
• Support discontinuous output. Could be achieved by adding a separate template argument and perform a similar mechanism as currently done for the lagrange wrapper (i.e. add one more wrapper layer around the stored grid...). EDIT: will be done in a separate MR.
• Add support for vol-var output as in VTKOutputModule. I suggest to derive from this writer and expose an additional setVolVarField function, because for vol var output we need more things (e.g. GridVariables, X, etc...). By adding it as additional derived class we can keep simple output "simpler". EDIT: will be done in a separate MR.

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