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.
This way, the dot(1) program won't generate errors like
Badly delimited number 1_ splits into two tokens
when presented with a well name like 'INJE-1_N'.
