Allow for custom datatype names in votable

This should allow end users to workaround VOTable files that contain
invalid datatypes.

Also adds two datatypes seen in Vizier tables: "unsignedInt" and "unsignedShort".
Update docs about datatype mappings.
Add changes entry.
Merge pull request #2675 from mdboom/votable/custom-datatype-mapping

VOTable: mask out invalid data types?