hp.vis.vis2d.show_scatterer_slices - not sure if this still works

[anna-wang]

I'm not sure hp.vis.vis2d.show_scatterer_slices works/I don't know if there's a different way of calling it now. When I type the following I get a bunch of black slices:

import holopy as hp
from holopy.scattering.scatterer import Ellipsoid

test = Ellipsoid(n = 1.585, r = [.4,0.4,1.5], center = [10,10,20])
hp.vis.vis2d.show_scatterer_slices(test,0.1)

I also get an error the first time I try this:

colorbar.py:829: RuntimeWarning: invalid value encountered in divide
  z = np.take(y, i0) + (xn - np.take(b, i0)) * dy / db

example slice:

[tdimiduk]

There was a bug in ellipsoid, PR #9 should fix it. Check out the PR and if it looks good, please merge it.