Fix x, y order with numpy.mgrid usage

As discussed in #2038 with @larrybradley and illustrated [here](http://nbviewer.ipython.org/gist/cdeil/9134816), the correct way to use [np.mgrid](http://docs.scipy.org/doc/numpy/reference/generated/numpy.mgrid.html) to create coordinate arrays `x` and `y` is ``y, x = np.mgrid[0:100, 0:200]``.
For [np.meshgrid](http://docs.scipy.org/doc/numpy/reference/generated/numpy.meshgrid.html) it is `X, Y = np.meshgrid(x, y)` for the default `indexing='xy'` order.

This PR fixes a few cases in the modeling tests and docs where the wrong order was used.