shear positions

[ismael-mendoza]

When shear is applied positions should be shifted as well to be very precise (small effect), see descwl-shear-sims for an example:

https://github.com/LSSTDESC/descwl-shear-sims/blob/98bf8cfbd68f5b2b075f78bcf56fc80e974584cc/descwl_shear_sims/sim.py#L451