this means getting rid of the Opm::filesystem namespace
and directly using std::filesystem, as well as dropping
some of the compat code for the tr filesystem.
we still need the FileSystem.hpp/cpp files though as unique_path
is not part of std::filesystem.
since we still support g++-7, where filesystem is marked experimental,
we introduce a wrapper header and expose the namespace to use
as Opm::filesystem.
for gcc we unconditionally link with libstdc++fs in the python bindings.
the setup.py stuff links as c code, not c++ code, so it is not
automatically added on any gcc version. this might prove unportable
later.
calculated and stored as private data members in EclipseGrid.
The API for the class is unchanged except for some minor changes for exportACTNUM, exportZCORN and exportCOORD.
These changes have triggered some very few modifications in the opm-grid and opm-simulators repo.
