Implement physically motivated PSF models

[AnthonyHorton]

Currently the only PSF model is a Moffat function. With just 3 parameters (normalisation, width & 'shape') this can approximate real instrument PSFs pretty well, however there is no simple connection between the width & shape parameters of the model and physical parameters of the imaging system. This limits both the ability to predict the PSF of an instrument and the ability to infer information about an instrument from measured PSFs.

A rigorous approach to PSF modelling is beyond the scope of Gunagala, that's for end-to-end systems engineering models. Physically motivated PSF models can be introduced while retaining the parametric performance model nature of Gunagala, however. Components of these PSF models could include:

1. Imager aperture -> Airy disc (generalised to include central obstruction)
2. Kolmogorov/van Karman atmospheric turbulence spectrum -> seeing disc & wings
3. Optical aberrations.
4. Scattering from atmospheric aerosols.
5. Scattering from optics.
6. Tracking/guiding errors.

1-5 are wavelength dependent to varying degrees, some sort of band pass averaging will be required. For most instruments 1 and/or 2 will dominate, and simple models for 3, 4, 5 & 6 will be sufficient.