add option to include object poisson noise

[esheldon]

I recall I had an issue making this work but I can't remember the details

[esheldon]

The reason I didn't do this is that I think the flux of an object in the sims, they way we are doing them, would not correspond to a certain number of photons.

It may be that for weaklensingdeblending sims we could do poisson if we use them correctly, but we would also need to be careful about the additional noise added later.