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.

tests/parser/integration/BoxTest.cpp

@@ -22,7 +22,7 @@
 #include <boost/test/unit_test.hpp>
 #include <boost/test/test_tools.hpp>
 
-#include <boost/filesystem/path.hpp>
+#include <opm/common/utility/FileSystem.hpp>
 
 #include <opm/parser/eclipse/Deck/Deck.hpp>
 #include <opm/parser/eclipse/Parser/Parser.hpp>
@@ -38,7 +38,7 @@ inline std::string prefix() {
 
 inline Deck makeDeck(const std::string& fileName) {
     Parser parser;
-    boost::filesystem::path boxFile(fileName);
+    Opm::filesystem::path boxFile(fileName);
     return parser.parseFile(boxFile.string());
 }