Guidelines on plate solving

[lgrcia]

Write guidelines on the parameters to use when doing plate solving.

There is a trade-off between the number of stars to consider against the number of queried stars to match (more stars make the match slower). For example, we retain the n brightest Gaia stars in a field s times larger than the expected field of view (using image size and pixel known plate scale), and we compare these stars to the m brightest detected stars in the image. All these parameters play an important role, especially since brightness of Gaia stars is sorted from phot_g_mean_fluxes that may not match the relative brightness of the stars in a given observation with different wavelength sensitivity.

Some guidelines would be useful (see #58)

[lgrcia]

Probably not useful anymore thanks to #106 (twirl 0.4 being more robust)