Updating slope-aspect raster grid routines

Added a new, faster (? - not looped) method to find the local slope and
aspect of a core node in the grid. This should now play nicely with
grids with boundary nodes that aren’t perimeter nodes.
Also, updated diagonal_lists and neighbor_lists so they’re now
internally consistent: both now use BAD_INDEX_VALUE, and boundary nodes
can now have neighbours, as long as they’re core nodes.