Detect cost rasters with only zero values

The system will not work when all cells in the cost raster are of zero value. It is relatively trivial to trap and raise an error when such rasters are passed to the system, so it should be added.

Cost rasters can validly have zero values, but in most cases they will not be of use. Mixed value rasters with zeroes are harder to detect (unless they are integers). It is best left to the user to decide in such cases, though.