Return pat_ids for records which have no saved form (i.e. no data)

[PatrickRWright]

Is your feature request related to a problem? Please describe.
Sometimes data entry personnel may register records which are empty. There are likely useless but they affect the overall number of registered participants. Thus it may be handy to know which records have no saved forms.

Describe the solution you'd like
Probably a function that checks which records in the cn table have no records anywhere else.