Split off separate functions into modules

[cbassa]

Currently process.py performs all processing functions: initial astrometric calibration, star finding, astrometric calibration, computing satellite predictions, extracting satellite tracks and storing the results. These functions should be split off into separate modules which can be executed individually, but also be called from a master script similar to process.py. The individual functions should have an option which forces the recomputation of results, ideally overwriting earlier results (this requires removing some append steps).