add warning message when unit conversion is enabled

src/potential_file_reader.cpp

@@ -174,7 +174,9 @@ TextFileReader *PotentialFileReader::open_potential(const std::string &path) {
         }
       }
     }
-
+    if (unit_convert != utils::NOCONVERT)
+      lmp->error->warning(FLERR, fmt::format("Converting {} in {} units to {} "
+                                             "units", filetype, units, unit_style));
     return new TextFileReader(filepath, filetype);
   }
   return nullptr;