Modification to compute relative relief

[pwernette]

I noticed that relative relief is currently computed along a single transect, which is different from how relative relief was initially proposed and affects its delineation ability. Relative relief should be computed over a given area, instead of along a transect. To compute it along a transect is neglecting potentially important information in the DEM cells adjacent to the transect itself. Once relative relief has been computed using a planimetric 2D window (ideally across 3 different spatial scales), then it is reasonable to apply thresholding to determine the location of dune features. While the distinction between computing relative relief within a 2D area versus computing relative relief along a transect may seem trivial, the results are significantly different, with the transect-based computation typically being an underestimate of dune feature locations.