/// @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 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 Main distance interface: given two collision objects, and the requirements for contacts, including whether return the nearest points, this function performs the distance between them.
/// Return value is the minimum distance generated between the two objects.