Sentinel-2 Importer Improvemente: download of individual bands

[anikaweinmann]

From #295 some TODOs are open one of these is the improvement of i.sentinel.download such that it can directly download individual bands: improve i.sentinel.download such that it can directly download individual bands (and required metadata) instead of the entire scene, (and maybe even cut it to the current region) and add these new functionallity to the Sentinel-2 importer