With the adoption of RFC-177, and its implementation in DM-5642, use of Astropy compliant unit strings is now enforced in afw tables.
A check for compatible units is done with every call to schema.addField and in the constructors of the following tasks:
Additionally compatible units are now used as defaults throughout the stack and in all associated test data.
Please be sure to pull the latest master for all packages.
For more information, and an exhaustive list of supported units in Astropy, see http://docs.astropy.org/en/stable/units/