Flag/remove unused value label sets

[arthur-shaw]

Problem

Value labels may exist in data without being attached to any variable. Such superfluous variable label sets can easily go unnoticed--not least since there's no easy method for identifying them and, if desired, removing them.

Implementation ideas

For collecting:

• Get the names of all labels via r(names) returned by label dir
• Get the list of variables that have this label via ds, has(vallabel label_name)
• Collect the names of labels that have an empty return from ds above
• Return these labels in r(names) (or other convetional name)

For removing:

• Pass r(names) to label drop