Estimate rgrd in sfr using upstream and downstream reaches (where available)

[wkitlasten]

Suggesting a new method to use upstream and downstream reach elevations in set_reach_slope.

Maybe change rch_len method to:

rch_len_grid: calc dz from slope of top model grid
calc rgrd as dz/rlen

and add new method:

rch_len_grid: calc dz from difference between upstream
rtop and downstream rtop
calc rgrd as dz/rlen

One potential issue is breaking scripts that rely on method="rch_len". Not sure how to handle the "deprication" of something like that. Otherwise I think it is pretty straightforward. Any blindspots I might be missing?