Add more validators for FITS column attributes

[embray]

Over in #3283 I added a new interface to easily validate user-supplied values for the various attributes that can be attached to a column in a FITS table (name, units, scaling, dimensions, etc.)

I only implemented a validator for column names (the TTYPEn keyword in the header), as proof of concept. But I left others unimplemented just for the sake of getting the initial changes in and moving on. This is a reminder to do the others as well--this will be useful even when (if not especially when) migrating FITS tables to the Table interface.