Refactor the extraction of a slice of an array

• Add a mpdaf.obj.objs.circular_bounding_box function to do this consistently
• Cube.subcube was missing one pixel at the edge of the extracted array if the extracted size was the same as the cube's shape. For this one, use astropy.nddata.overlap_slices which return the slices for the input and output arrays.
• Add tests for the objs module. (+ other minor changes)