r.watershed: Keep 0 accumulation with -a as 0 as opposed to null

[HuidaeCho]

This PR addresses #2003 by saving 0 accumulated cells as 0, not as null, for the -a flag. It does not change the default behavior without -a.

Its side effect is that cells outside non-null input are also saved as 0 (was null before this PR), which is in fact more consistent with the default output without the -a flag. I think with/without -a, null input cells should produce null output cells (TODO).

[metzm]

Looks good to me

[ar-siddiqui]

@HuidaeCho thanks for taking care of the original issue. Yes, I also noticed this as well that without -a flag original null cells are being converted to 0 value. This is lesser of a problem because the user can use a raster calculator this way (input raster * 0) + grass output to get the null values back.

Having said that can r.watershed do that internally as a last step to fix nulls being converted to 0? Perhaps this should be discussed in a separate issue.

[HuidaeCho]

@HuidaeCho thanks for taking care of the original issue. Yes, I also noticed this as well that without -a flag original null cells are being converted to 0 value. This is lesser of a problem because the user can use a raster calculator this way (input raster * 0) + grass output to get the null values back.

Having said that can r.watershed do that internally as a last step to fix nulls being converted to 0? Perhaps this should be discussed in a separate issue.

Check #2221.