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.
motivation: an upcoming parallel frontend to the field props manager.
templated functions cannot be virtualized, and thus having these exposed
would give a great chance of confusing the users in the downstream
code, where properties would be caught from the (potentially) empty
backend instead of from the frontend.
