Glad's is_in_active_grid is slightly inconsistent with logic elsewhere in CISM

This is a spin-off from #39 . I noticed that the conditional that determines the icemask in `is_in_active_grid` checks `usrf > 0`. But apparently some code elsewhere in CISM treats points with topg exactly equal to 0 as land. So, for consistency, is_in_active_grid should also consider these points with topg exactly equal to 0 to be land, and thus within the ice mask.

As discussed in #39 , this inconsistency is a problem for conservation, so it should definitely be fixed. However, it only impacts a small number of grid cells. As also discussed in #39 , changing this conditional to `usrf >= 0` doesn't work, because ocean grid cells have `usrf == 0`, so that change led all points in the CISM domain to be considered to be within the ice mask. @whlipscomb suggested a possible change like `usrf > 0 .or. topg >= 0`, but we want to check to make sure that really is the correct conditional, consistent with what is used elsewhere. Ideally, we would use an existing mask variable that is consistent with what is used elsewhere.