`pseudo_gravity` and `pseudo_electrostatic` packings should have `shape` or `im` as arguments

[jgostick]

This is an api breaking change, so no rush.

[jgostick]

I am no longer in favor if this im_or_shape argument. Probably these functions should accept one or the other, like def func(im=None, shape=None, **kwargs). This means that all other arguments must have defaults, but I think this is ok. Even for the sphere radius, r=5 is not that bad of an assumption for a first pass.