Skip parts of PSF modeling which are not being tested

This ticket is from the series ``Improvements to the PSF 2d modeling, April 2019 brainstorming''.

The code currently does the full procedure no matter what you are testing:
1. Create pupil
2. Illuminate pupil
3. Apply wavefront aberrations
4. Do FFT**2
5. Do post-processing (add scatter, locate center...)

The code should be able to skip certain steps if we are not testing some parameters. Most obvious is the pupil - no need to recreate pupil over and over again if it does not change during the fit.