Initial release

Initial release of the pygsr code, which is a simplified version of an astrometric sphere recovery from space observations by the ESA Gaia mission, including a relativistic treatment of light propagation and aberration.

The code currently gets a set of observation epochs and auxiliary data (star catalog, Gaia ephemeris and attitude, Sun ephemeris), simulates observations by applying both catalog errors and observation noise and solves for a set of astrometric parameters (positions, parallaxe and proper motions).
The main classes implemented in the code are: a "star" class and an "obs_eq" class, including all methods required to set-up and solve the least-square problem from data related to each star. No global parameters (e.g., attitude corrections, ppn-parameters) are included.

Tested only in specific cases to develop an exercise for the Gaia Summer School 2019 in l'Aquila, Italy.