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.
This commit switches a set of OPM-Common's unit tests away from
using direct calls to libecl functions and into using base types
from OPM-Common itself (along with Boost.Filesystem).
In particular summary related queries are replaced by calls to ESmry
member functions (wrapped in libecl-like interfaces to minimise code
changes). We disable checks on unit strings since ESmry currently
does not have a way of associating those with individual variables.
