slope and aspect calculations test for neighbor existence with "BAD_NODE_VALUE"

raster.py functions written by K Barnhart to calculate slope and aspect have been modified to not use cells that have "BAD_INDEX_VALUE" for the neighborhood. Before this fix, model would break if there were open boundaries (and thus cells without a full set of 8 neighbors).

this does not fix any of the problems noted in the text about the aspect calculation not working correctly. KRB will look into this when she has a chance.