changed: drop gcc7 compatibility

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.

test_util/EclRegressionTest.cpp

@@ -30,13 +30,13 @@
 
 #include <opm/output/eclipse/RestartIO.hpp>
 
-#include <opm/common/utility/FileSystem.hpp>
 #include <opm/common/ErrorMacros.hpp>
 #include <opm/common/utility/numeric/cmp.hpp>
 
 #include <algorithm>
 #include <chrono>
 #include <cmath>
+#include <filesystem>
 #include <iomanip>
 #include <iostream>
 #include <set>
@@ -998,7 +998,7 @@ void ECLRegressionTest::results_smry()
             }
         }
 
-        namespace fs = Opm::filesystem;
+        namespace fs = std::filesystem;
         std::string rsm_file = rootName2 + ".RSM";
         if (fs::is_regular_file(fs::path(rsm_file))) {
             auto rsm = ERsm(rsm_file);

test_util/summary.cpp

@@ -18,6 +18,7 @@
 */
 
 #include <cmath>
+#include <filesystem>
 #include <iomanip>
 #include <iostream>
 #include <tuple>
@@ -99,7 +100,7 @@ int main(int argc, char **argv) {
     std::unique_ptr<Opm::EclIO::ExtESmry> ext_esmry;
 
     std::string filename = argv[argOffset];
-    Opm::filesystem::path inputFileName(filename);
+    std::filesystem::path inputFileName(filename);
 
     if (inputFileName.extension()=="")
         inputFileName+=".SMSPEC";