Fix the subsetting tool assumptions regarding 1:1 `wb` and `cat` designations

[jameshalgren]

See comments here:

data_access_examples/subsetting/subset.py

Lines 140 to 141 in 9101c4c

	# As long as these remain 1-1 this works, but that may not always be the case
	# FIXME in fact this isn't true at all, there can be catchments with no FP, and FP with no catchment

and here:

data_access_examples/subsetting/README.md

Lines 13 to 15 in 9101c4c

	# Note
	A current shortcut is being used to map `wb` and `cat` ids that isn't a valid assumption, and will be fixed in the future.
	This means you might get a subset that isn't topologically consistent, so use at your own risk.