Corrected modification of edges.

Bug in code was pulling wrong map node for tile coordinates. Also
changed routine to modify edge only if destination is map node. I may
have to reconsider this, but worker now correctly moves in a way that
avoids colored map tiles.
Merge branch 'issue17' into develop

Corrected problems with code assigning new weight to edges in
AtlantisMap. We may have to revistit this once the offset is considered
in order to make sure that the ground entities correctly avoid terrain
obstacles.