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 is an effort to improve build performance. Several includes
scattered across the project are either unused or partially used (i.e.
just used to import a type name, not depending on the actual contents of
the header file).
Replaces a lot of these includes with forward declarations.
this is just the result of
```
find -iname "*.[ch]pp" | xargs sed -i "s/ *$//"
find opm/parser/share/keywords -type f | xargs sed -i "s/ *$//"
```
so if it causes conflicts with other patches, the others should get
priority. The rationale behind this patch is that some people tell
their editor to remove white space which leads to larger than
necessary patches...
