A grid which can manage non-ponctual objects

The current grid (StelGrid class) can handle object with a position defined by a point. It would be very useful to have a new implementation of a grid which manages object with an area, and allowing to make fast lookup of all objects intersecting a given convex.
The area of an element can be defined by a convex for simplicity.

One way to implement it with good performance would be to still reference objects by a single point, but make the queries with a margin larger than the object maximum radius. The objects just need to be stored in several grids, one for each radius.

Skills needed: C++, spherical geometry
Difficulty: challenging