Clean up true 2D support for cameras/SLMs with non-square pixels.

[cpanuski]

Many returns (e.g. computed PSF sizes) are averages of the results in x and y dimensions; results should instead be vectors with results for each dimension and handled accordingly.

[ichristen]

(We discussed separately and noted that the full picture would actually be a 2x2 matrix including shear. The question is whether to add this now or add it reactively when someone needs it.)