[Therion] Revise command: how to remove persons later on

[Bruce Mutton]

> I'm not sure that the progranm should be growing features to cater for
data revision without actually revising the data.

I agree with the sentiment that we should resist 'feature bloat', however
Therion already has a 'revise' capability.  I do not understand it well, and
that has been frustrating over time, as there have been more circumstances
that I have not been able to get revise to work, than vise-versa.

I like the idea of the ability to 'equate' or 'merge' names (maybe
'merge-name' as 'merge' is perhaps too generic a term) and to be able to
revise a dataset at compile time to remove names.  I image there are valid
reasons to want to do this.

Bruce