Public access to reference data for units and aspects

[rjyounes]

The gist 13.0.0 release does not include any units, and a handful of aspects needed for restrictions. Users need both standard units and a way to get additional units and aspects from our reference data. This might involve two different solutions.

Consider immediate post-release of gist 13.0.0, short-term, long-term solutions.

[uscholdm]

See also issue #1035 - what to do when all the units disappear from gist.