Make a PSF object type which is a call to a PIFF object

[cwwalter]

Especially for things like SSI, it would be good to be able simulate a PSF based on an the real PSF in an image at an arbitrary location.

@rmjarvis suggests that this could be an input type with the code for the module located in the piff directory. The class could run piff on an image used as a base (related to #109) and then return a piff object which could be used to create a PSF at an arbitrary location. I'm not sure if piff objects are typically saved with processed exposures but it could use that instead if it existed.

[beckermr]

This is already done here: https://github.com/LSSTDESC/seacliff/blob/main/seacliff/rubin_psf.py

[beckermr]

As I said before, seacliff is potentially a very natural home for SSI. imSim can pull in seacliff modules as well.

[beckermr]

I have also done the WCS:

https://github.com/LSSTDESC/seacliff/blob/main/seacliff/rubin_sky_wcs.py

and the noise:

https://github.com/LSSTDESC/seacliff/blob/main/seacliff/rubin_noise.py

[cwwalter]

Great. We should have you come and talk about seacliff in a meeting (I'm still not exactly sure what it is) and also help us understand how we could use this module from imSim.