Use `terrain_only` in testFloatingBaseDynamics.
`getTerrainContactPoints` has collision_group arg
Add body_idx arg to RBM/getTerrainContactPoints
Exclude world when finding terrain contact points

... because world-fixed objects don't collide with the ground.
Fix getTerrainContactPoints in RBSphere

Return an empty array for non-zero radius spheres.
Add getTerrainContactPoints passthrough in TSRBM
Add terrainContactPositions method to [TS]RBM

Move contactPositions[Jdot] to dev
Remove contactPositions passthroughs from TSRBM
Remove contactPositions[Jdot] from dev.
Update tests to use terrainContactPositions
Use body_idx in terrain part of collisionDetect

If `active_collision_options.body_idx` is non-empty, only check the bodies
listed against the terrain.
Populate cpp RigidBody contact_pts

`constructModelmex` calls `getTerrainContactPoints` and passes those points
along to the c++ `RigidBody` objects.