[feature request] add option to use D-infinity pointer grid in ElevationAboveStream

[markwang0]

First, thank you for sharing this fantastic package. I have been slowly replacing some of my GRASS GIS and TauDEM workflows with the open core release of Whitebox Tools.

One feature I am missing is the ability to calculate Height Above Nearest Drainage (HAND) using a D-infinity pointer grid. This is something we currently accomplish with the TauDEM tool DInfDistanceDown (and flag -m ave v), but would like to replace with Whitebox Tools. I believe that ElevationAboveStream uses a D8 pointer grid, but please correct me if I'm wrong.

We use D-inf based HAND for flood inundation mapping because the results don't look quite as good with D8 HAND. Would it be possible to add an option to ElevationAboveStream to choose between a D8 or D-inf pointer?