cleanup: use constants for RivEdge table (in MapGenerator)

replace hard-coded numbers with constants and constants plus offsets

git-svn-id: https://micropolis.googlecode.com/svn/trunk/micropolis-java@889 d9718cc8-9f43-0410-858b-315f434eb58c
cleanup: do not use TileConstants. prefix when not needed

git-svn-id: https://micropolis.googlecode.com/svn/trunk/micropolis-java@890 d9718cc8-9f43-0410-858b-315f434eb58c
cleanup: use named constants in isRiverEdge

instead of hard-coded numbers

git-svn-id: https://micropolis.googlecode.com/svn/trunk/micropolis-java@891 d9718cc8-9f43-0410-858b-315f434eb58c
tool-effect: treat tiles outside of city boundary as dirt

git-svn-id: https://micropolis.googlecode.com/svn/trunk/micropolis-java@892 d9718cc8-9f43-0410-858b-315f434eb58c