PVTUReader: use std::experimental::filesystem library

#pragma once

#include <experimental/filesystem>

#include <TNL/Communicators/MpiCommunicator.h>

#include <TNL/Meshes/Readers/VTUReader.h>

#include <TNL/Meshes/MeshDetails/layers/EntityTags/Traits.h>

   std::string

   getSourcePath( std::string source )

   {

      // TODO: use proper path library

      std::vector< String > parts = String( fileName ).split( '/' );

      parts.pop_back();

      parts.push_back( source );

      std::string result;

      for( auto p : parts )

         if( result.empty() )

            result += p;

         else

            result += "/" + p;

      return result;

      namespace fs = std::experimental::filesystem;

      return fs::path(fileName).parent_path() / source;

   }

#ifdef HAVE_TINYXML2