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. 

It's parsing the value of the '_id' property of the self.provider_data dictionary.

That dictionary was created upstream by xmltodict's parsing the XML in the fetcher and serialized as JSON.  It was then posted to the mapper Akara endpoint and deserialized.

The regex gets the part of the string at the end of the line after the last "/" character and uses a Python named positional parameter.