Add a system whereby collections of units can be enabled after the fact.
Add additional units in CDS/Vizier format
Move % and --- to the cds.py unit collection
Make Crab completely CDS-specific
Fixups after rebase
Fix doc build by using an approach that will get unicode attributes on modules correctly on Python 2.x
Merge pull request #1468 from mdboom/unit/cds-extra-units

Update CDS units