add support for multiple flow dirrections
Merge remote-tracking branch 'upstream/master' into hydrology
add test case for multiple flow dirrections (incomplete)
Merge remote-tracking branch 'upstream/master' into hydrology
Merge branch 'hydrology' of github.com:Markko/geotrellis into hydrology
finish accumulation test for multiple flow dirrections
Merge pull request #612 from Markko/hydrology

add support for multiple flow dirrections