Upperlimit estimations, updated error estimation and other quality of life improvements

Upperlimit estimations: implemented and documented additional methods of estimation : from raw distribution of detected sources, from the faintest source with SNR>5, from artificial star injections and recovery (only partially developed, not implemented yet) Allowed for the use of custom masks of user-specified size, optionally centered on transient coordinates

Error estimation: Reworked the error estimation and propagation formulas, now taking into account the effective gain of each instrument (some default values are stored)

Small bugfixes and corrections