Applicable to CivilStorm, StormCADD, and PondPack.

Sometimes we only want to calculate the runoff generated by a catchment area. However, catchment areas do not calculate runoff unless a outflow node is specified. White it's not terribly difficult to make a dummy outfall node, it would be really nice for the drainage area to be able to compute the runoff and simply generate an error message that there is no outflow node specified.