Sentinel-2 Importer Improvemente: extent/bbox

[anikaweinmann]

From #295 some TODOs are open one of these is the implementation of the extent (bbox): import processes are automatically put before grass (or other) processes in a process chain. This means that no region can be set via a grass module before running the import. Limiting the import data to a certain extent has to be done via the extent parameter of the importer. This still needs to be implemented.