Use test-installed-landlab.py to run tests.
Added note about using *bool* type.
Import HexModelGrid in doctests.
Import RasterModelGrid in doctests.
Fixed ma in calculate_gradient_across_cell_faces.

Fixed
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* Use the masked array in calculate_gradient_across_cell_faces. This
  fixes a merge error where the regular array was used in the subtract
  ufunc instead of the masked array.
* Fixed related doctests that used the masked arrays.