v.category has [output=map] as optional, yet if you leave it out you get
an error.
I expect if not given v.category should try and update existing vector?
task:
I have made a new vector lines map with 'v.distance out=' and have to
give features a cat number before giving it a table to upload lengths
into (v.to.db) for v.univar.sh analysis.
optional-but-broken or shouldn't-be-optional?
Hamish