NDR runoff proxy index depends on watershed extent

[emlys]

@phawthorne raised this issue:
The runoff proxy index is calculated as $RPI_i = RP_i / RP_{av}$ (dividing runoff proxy by average runoff proxy within the watersheds). This is problematic because average runoff proxy $RP_{av}$ changes depending on which pixels/watersheds are included in the raster. @phawthorne said they have even considered inserting fake pixel values into the RP raster in order to change the $RP_{av}$. We could expose the $RP_{av}$ parameter and allow the user to specify it instead.