With this commit the PYACTION keyword is changed, instead of embedding the
Python code directly in the .DATA file the keyword now points to an external
file which is loaded verbatim into the PyAction keyword.
In addition the PYACTION keyword has now got a name and a string indicating how
many times it should run.
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.
