implement shear in z slices

[esheldon]

But we should only do this if we plan to use these sims for calibration

[NiallMac]

But we should only do this if we plan to use these sims for calibration

@esheldon I'm not really sure what the point you're making is here. But probably I'm just not understanding in general what these sims are for. If we want to use them to test the effects of blending between objects at different redshifts (where "test" here could be called validation or calibration, I don't think there's much of a difference between these two practically), then I think we'll want to allow for z-dependent shear. If that's not in the remit of this sims code, then I'd like to understand that remit a bit better.

[esheldon]

I've changed my mind about this. I think it would be very good to have it and we would learn from it.

As for the remit: the realism needed for calibration is considered a secondary goal because we don't currently have the knowledge of how to do it. We would need contributors with that knowledge.

[beckermr]

@esheldon My student just got two shear slices working in our old mdet sims code FWIW.

[NiallMac]

(see #128)