changed: use std::filesystem instead of boost::filesystem

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.

cmake/Modules/OpmCompilerChecks.cmake

@@ -91,3 +91,9 @@ CHECK_CXX_SOURCE_COMPILES("
    };
 "  HAS_ATTRIBUTE_DEPRECATED_MSG
 )
+
+if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+  if (CMAKE_CXX_COMPILER_VERSRSION VERSION_LESS 8.0)
+      list(APPEND ${project}_LIBRARIES stdc++fs)
+  endif()
+endif()