BUG: Catchment outlet on raster boundary shouldn't raise an error.

* Reported by hannes.betrieb on the mailing list along with a
  suggested fix
  (https://groups.google.com/forum/?fromgroups=#!topic/pytopkapi/LMWBU0u8rZg)
TST: Add a test for Github issue #7
Merge branch 'cell-connectivity-bug'