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. 

@anarchivist I think the split is better, in this case, as long as there's an assignment that says 'identifier =' so that it's clear what it is.

I'm thinking:  ```identifier = id_prop.split('/')[-1]```

@no-reply I'd normally agree with creating a function, but my question is whether it makes the code easier to follow and if the extra height in lines would be worth it, since it will work as a one-liner with the assignment above.