Make ST_Accessibility accept a single destination

Note that

    ST_Accessibility('cormen_edges_all',
        'directed - edge_orientation', 'weight', '5')

is practically equivalent to reversing the graph and using
ST_ShortestPathLength as follows:

    ST_ShortestPathLength('cormen_edges_all',
        'reversed - edge_orientation', 'weight', 5)

with the following columns switched:

                ST_Acc  |  ST_SPL
          ----------------------------
                SOURCE <=> DESTINATION
          CLOSEST_DEST <=> SOURCE