Refactoring polygon_query() and bounding_box_query() #255

TODOs:

• use sjoin when querying by multiple polygons at the same time.
  *EDIT: I implemented sjoin and tested the support for MuliPolygon. I have removed the possibility to query by multiple polygons, as the behavior that was implemented was equivalent to querying by a single Multipolygon.
• unify APIs interface