Add option to replace (instead of just add) cats with v.category

[needelsd]

Add an option to v.category option=replace that overwrites the cat values in a vector instead of only adding a cat to objects that do not already have a cat.

Equivalent to

v.category input=input_vector output=tmp option=del v.category input=tmp output=output_vector option=add g.remove vect=tmp

Personally, I would also appreciate a flag [-r]?? or other mechanism to overwrite cats in the original vector (allow output=input).