Implement PSF model with a physical layer

[jeipollack]

To address various issues in the modelling like centroid problem, chip misalignments, and including a prior a new PSF model class needs to be added to WaveDiff >v2.0.x.

It's implementation is illustrated in the far left in this image:

The PSF model class is in version 1.0 but requires refactoring to be included in v2.0.x

[jeipollack]

Hi @tobias-liaudat, I need a reminder about the implementation on padding the Zernikes. I see there is already a method for padding L321, which is implemented in the method compute_zernikes at L484 and executed in the call class method.

[jeipollack]

Note, I am still making changes to psf_model_physical_polychromatic.py module as I begin to write unit tests and think about interfaces with the other parts of the pipeline.