More documentation on Fluxes

[lgrcia]

Amazing review @szunigaf, thanks!

Here are some documentation issues that needs to be addressed ASAP:

• Update the photometry tutorials to avoid reusing the *calibration sequence
• Update the photometry tutorials on how measurements are extracted in GetFluxes
• Update the photometry to show how prose 2.0.0 diagnostic plots can be reproduced (waiting for a detailed issue about that @szunigaf)
• On top of the Fluxes docstring, give more context on the shape of fluxes and how it translates on other attributes (like weights.
• On top of the Fluxes docstring, add a note about inplace vs. return methods
• Extensively document docstrings of Fluxes.autodiff, specify what attributes it changes and that it calls set_aperture...
• Documentation of Fluxes.flux is very bad
• Badly displayed documentation for Fluxes.fluxes, keep it simple and add more details in Fluxes main docstring

[szunigaf]

Some features that I miss from prose 2 are the one included in the obs object like:

Comparison stars and Systematics.

Or the summary, specially the obs.plot_radial_psf part (lower left corner).

Last but not least, the ref.plot_psf_model().

[lgrcia]

The first comparisons and explanatory measurements plot are now described in the exoplanet transit tutorial (part of #116)