Added function getMeshReader

It is useful especially when one wants to load mesh functions via the
readPointData or readCellData methods when the mesh was already loaded.

src/TNL/Meshes/Readers/getMeshReader.h

+/***************************************************************************
+                          getMeshReader.h  -  description
+                             -------------------
+    begin                : Nov 7, 2020
+    copyright            : (C) 2020 by Tomas Oberhuber et al.
+    email                : tomas.oberhuber@fjfi.cvut.cz
+ ***************************************************************************/
+
+/* See Copyright Notice in tnl/Copyright */
+
+// Implemented by: Jakub Klinkovský
+
+#pragma once
+
+#include <experimental/filesystem>
+
+#include <TNL/Meshes/Readers/NetgenReader.h>
+#include <TNL/Meshes/Readers/VTKReader.h>
+#include <TNL/Meshes/Readers/VTUReader.h>
+#include <TNL/Meshes/Readers/PVTUReader.h>
+
+namespace TNL {
+namespace Meshes {
+namespace Readers {
+
+std::shared_ptr< Readers::MeshReader >
+getMeshReader( const std::string& fileName,
+               const std::string& fileFormat )
+{
+   namespace fs = std::experimental::filesystem;
+   std::string format = fileFormat;
+   if( format == "auto" ) {
+      format = fs::path(fileName).extension();
+      if( format.length() > 0 )
+         // remove dot from the extension
+         format = format.substr(1);
+   }
+
+   if( format == "ng" )
+      return std::make_shared< Readers::NetgenReader >( fileName );
+   else if( format == "vtk" )
+      return std::make_shared< Readers::VTKReader >( fileName );
+   else if( format == "vtu" )
+      return std::make_shared< Readers::VTUReader >( fileName );
+   else if( format == "pvtu" )
+      return std::make_shared< Readers::PVTUReader >( fileName );
+
+   if( fileFormat == "auto" )
+      std::cerr << "File '" << fileName << "' has unsupported format (based on the file extension): " << format << ".";
+   else
+      std::cerr << "Unsupported fileFormat parameter: " << fileFormat << ".";
+   std::cerr << " Supported formats are 'vtk', 'vtu', 'pvtu' and 'ng'." << std::endl;
+   return nullptr;
+}
+
+} // namespace Readers
+} // namespace Meshes
+} // namespace TNL