Avoid negative values for Other Unimproved #2360

Consider whether Surface Water is too high and if it will exceed 100%
when added to Total Improved. If so, then we redefine Surface Water as
what is left from 100% - Total Improved. This has a direct consequence
on Other Unimproved which will become 0 instead of negative values.
Custom Surface Water threshold for Belarus #2360
Surface Water for rural is equal to urban for TFYR Macedonia #2360
Custom Surface Water threshold for United States of America #2360
Update computing doc to include Filter-Geoname rules