Add three dynamic-variant scenes: bimanual catch, two-ball catch, multi-bin throwing

[Manas-arumalla]

Follow-ups to the catching and throwing scenes (#43):

catch_bimanual_scene.xml: a ball is thrown toward either side of the
robot, and whichever arm can reach it best makes the catch. Grasp welds
for both arms, same weld-based velocity-matched convention as before.
catch_twoball_scene.xml — Two identical balls thrown at the same time,
one toward each arm; each arm catches one. Welds exist for every arm/ball pair since the assignment is decided at run time.
throw_multi_scene.xml — five balls on a small side table and five
narrow bins across the throwing envelope, for repeated throws to
distinct targets. The bins sit on the floor.