Added grid_flow_directions, disabled some doctests.

Added
-----
* New function grid_flow_directions calculates flow directions on a
  grid. This is a simplified version of the more general
  `flow_directions` function.

Changed
-------
* Turned off some doctests in `flow_directions` as they were not working
  and I couldn't figure out how to fix them. Perhaps they were even old
  tests that aren't even supposed to work any more.