improve geometry handling in search queries

[johntruckenbrodt]

This improves several aspects of scene search using spatial geometries:

• scene_select: combine all Polygon geometries into a single MultiPolygon and execute only one search query (instead of one for each polygon); this may drastically reduce search time for large AOIs
• STACArchive.select: search using the actual geometry of vectorobject and not its bounding box (in line with johntruckenbrodt/pyroSAR#296)
• introduce checks to the search methods (STACArchive.select, asf_select, ASFArchive.select) to only allow single-feature Vector objects (in line with johntruckenbrodt/pyroSAR#316))