Add content on PSF modelling methods

[tobias-liaudat]

• Add content on the different PSF modelling methods with entasis on non-parametric methods.

• Decide on how should this information should be structured.

[sfarrens]

Add new "part" for "Point Spread Function"

[aboucaud]

In the meantime, the repo structure has changed much.
Remember to pull the latest changes from the develop branch and look at the latest _toc.yml to know where to put your content.

[fadinammour]

When introducing the PSF, to give a grasp of its physical meaning we can apply different sizes of PSF on two points (close to each other) until the two point are not distinguishable anymore.

[aboucaud]

@fadinammour that's a good point.
To put it more in context, I would suggest using two simulated stamps of the same crowded field (including stars), as seen from a space telescope and a ground-based one.

[tobias-liaudat]

Nice gif for showing the speckle appearance of atmospheric PSFs with short exposures.

http://userpages.irap.omp.eu/~jprieur/pisco-fr/pisco/figs/a8035_movie.gif