Sentinel-2 Importer Improvemente: r.null or i.zero2null

[anikaweinmann]

From #295 some TODOs are open one of these is to correct null value:
When importing a scene, i.sentinel.import reprojects the map if it does not match the current projection. This may interfere with 0-values, that should be treated as NULL-values and removed via r.null or i.zero2null. This problem should probably be solved in r.import