Add additional prefixed units in CDS format

This is a follow-on to #1169, in which @RayPlante rightly points out that the CDS format needs to allow prefixes on virtually everything.

There were a few namespace clashes.  The CDS spec doesn't seem to address how those should be handled, but the VOUnit spec does say this, which seems like a good rule of thumb to follow:

```
Recognising a known unit takes priority over parsing for preﬁxes. Thus 
the string Pa represents the Pascal, and not the peta-year, and the string mol
will always be the mole, and never a milli-‘ol’, for some unknown unit ‘ol’.
```

Something to consider: This grows the unit namespace even further.  An alternative to this would be to handle the prefix parsing in the CDS parser so that the units package doesn't have to grow these new units.  The downside of that is that the user will have to remember whether units exist in the units namespace or only in the CDS format.