Flaky VoronoiModelGrid test on mac

[mcflugen]

The doctest for the VoronoiModelGrid.save method has become somewhat flaky recently.

The doctest is the following,

>>> from landlab import VoronoiDelaunayGrid
>>> import numpy as np
>>> import os
>>> x = np.random.rand(20)
>>> y = np.random.rand(20)
>>> vmg = VoronoiDelaunayGrid(x,y)
Traceback (most recent call last):
...
ValueError: cannot reindex or align along dimension 'cell' because of conflicting dimension sizes: {20, 21}

Apparently some combination of random x and y coordinates cause the save function to fail.

It may make sense to alter to doctest to use non-random coordinates but then add some unit tests that identify and test the underlying issue.