geometry buffering for minimum overlap

[johntruckenbrodt]

For geocoding, the processor first selects all MGRS tiles in a particular UTM zone with a SAR scene. The bounding box of the selected tiles is then used as subset for geocoding only parts of the scene. It can happen that the selected subset is too small and SNAP trows an error. This is more likely to occur near the poles where up to nine different UTM zones were observed to overlap with one EW scene.
With this PR, a new function is introduced to buffer the subset geometry to have a minimum overlap of 1% with the scene's footprint to ensure a sufficiently large subset for geocoding.
This is handled by functions ancillary.buffer_min_overlapand tile_extraction.aoi_from_scene. The latter was outsourced from code in snap.process.