Add small set of unit reference data in the gistCore repository

[rjyounes]

These would be the common units currently in gistCore, but now in the gistd: namespace.

[uscholdm]

I stroungly agree. That makes it much easier for people (in particular me) to understand exactly what the proposal is. It also means the community does not lose the information about units that have been around for years.

[rjyounes]

Closing - duplicates #1037.