Add CharlestonMapper for object mapping (#7651)

This should resolve https://issues.dp.la/issues/7651

College of Charleston (one of three South Carolina Digital Library providers) unexpectedly moved to a new repository system and we had to change the OAI endpoint URL, the 'set' specification, and the mapping of the 'object' property as a result.

CharlestonMapper extends SCDLMapper in order to perform special mapping of the 'object' property that is different than that of the other SCDL providers.  The included test describes the desired behavior. 

Maybe it would be more clear if it were a method, too?