#1563 Extend IsString so that a class may be defined to register true when queried
I need this for the helper class RefGeomEltAndMeshLabel, which is used for Ensight format and which role is just to read/write from disk a string, the class being there just to manage the interpretation of the string read or written.
I therefore needed it to be recognizable as a string, as in writing the output in Lua format I need to add quote around strings (associative policy related to Lua format for containers),