Add argument to fwa_watershedatmeasure to exclude fundamental watershed with measure

[joethorley]

Currently fwa_watershedatmeasure uses a hard to predict in advance algorithm to determine whether to keep, drop, cut or other the fundamental watershed that the upstream measure falls with in.

It would be useful if the user could specify whether to keep or drop the fundamental watershed or revert to the current algorithm.

[smnorris]

Good idea. Lots of open issues for this function, it could use some work.