BUG: Allow renaming of a column after mapping

Related #511

This should be merged before deciding on a less verbose way to rename things.
Re `DataDescriptor`, nevermind. I was grepping and I saw `_schema` defined, but I wasn't clear on what that was doing and how that's related to `Map`. I realized that they're not related. `_schema` for map is a possible input (same as `DataDescriptor`) but there's no relation in how the two properties are used.