/// @brief distance between two RSS bounding volumes
/// P and Q (optional return values) are the closest points in the rectangles, not the RSS. But the direction P - Q is the correct direction for cloest points
/// Notice that P and Q are both in the local frame of the first RSS (not global frame and not even the local frame of object 1)
/// @brief This is a special acceleration: BVH_model default stores the BV's transform in world coordinate. However, we can also store each BV's transform related to its parent
/// BV node. When traversing the BVH, this can save one matrix transformation.
/** \brief Compute the covariance matrix for a set or subset of points. if ts = null, then indices refer to points directly; otherwise refer to triangles */
/// @brief Compute the covariance matrix for a set or subset of points. if ts = null, then indices refer to points directly; otherwise refer to triangles