Fix bug in intersects_id
Fix bug in intersects_id

When returning locations, we must check that the list is non-empty because other grouping.unique_rows throws an error.

Moreover, the user is expecting numpy arrays as a return type instead of python lists, so return these instead of [],[],[].
dajusc/trimesh,mikedh/trimesh,mikedh/trimesh,mikedh/trimesh,dajusc/trimesh,mikedh/trimesh