Processing and simulation methods for iSCAT data

[vnmanoharan]

It would be nice to have HoloPy handle iSCAT data. We may be able to use PiSCAT to handle the processing. For simulating data, we have a model that works well with pymc for point particles. But for some experiments it would be nice to be able to simulate non-point particles and to handle the effects of the objective lens using the MieLens theory.

See discussions in #375 and #349 for helpful comments on what we might need to change.