Fixed doctest for _make_number_of_donors_array.

Fixed
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* Ensure that _make_number_of_donors_array returns an array of ints.
Fixed typo. Using numpy, not np.
Fixed docstring for flow_direction.
Fixed docstring.

Fixed
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* Fixed failed docstring test. The parameters for flow_direction had
  changed but the doctest wasn't updated. The doctest now passes but
  still need to make sure it's doing what it is supposed to.
Fixed doctests.
Fixed doctests.

Fixed
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* Changed a couple of the doctests to just look at array size and type
  since the arrays are composed of random values.
Fixed doctests.
Fixed doctests.
Removed file generated by unit tests.