Avoid std::filesystem

It has caused a lot of issues on systems with a default gcc <= 8.  PR AMReX-Codes#3523
was trying to the issue. But unfortunately it does not always work. So we
are going to simply avoid std::filesystem.

Tools/Plotfile/fgradient.cpp

@@ -6,7 +6,6 @@
 #include <AMReX_Vector.H>
 #include <cstdlib>
 #include <iterator>
-#include <filesystem>
 #include <sstream>
 #include <string>
 
@@ -129,7 +128,7 @@ void main_main()
  }
 
  if (outfile.empty()) {
- outfile = "grad."+std::filesystem::path(pltfile).filename().string();
+ outfile = "grad."+VisMF::BaseName(pltfile);
  }
 
  PlotFileData pf(pltfile);