Introduce deprecation warning on old template naming

cmake/ACGCommon.cmake

@@ -279,6 +279,7 @@ macro (acg_drop_templates list)
   foreach (_file ${${list}})
     if (_file MATCHES "T.cc$")
       set_source_files_properties(${_file} PROPERTIES HEADER_FILE_ONLY TRUE)
+      message("Deprecated naming scheme! The file ${_file} ends with T.cc indicating it is a template only implementation file. Please rename to T_impl.hh to avoid problems with several IDEs.")
     endif ()
   endforeach ()
 endmacro ()