Forbid NA as categorical level name

[davidorme]

It should be an error to specify NA as a possible categorical variable level. So something like this:

Alive;Dead;NA

should generate something like:

! Do not include the missing data code 'NA' as a level descriptor

rather than:

! Categories found in levels descriptor not used in data: 'NA'

This means no-one can call a level NA but that would be highly confusing!