sr_robot_commander can get wrong prefix in multi hand systems

[dg-shadow]

Except for the move groups right_hand/left_hand/right_arm/left_arm, sr_robot_commander uses a hand finder to get the prefix for the trajectory controller name. This means any commander instantiated in this way (e.g. for individual fingers) will always control the first hand, regardless of which hand is desired. There is now a solution possible using the (as yet unmerged) optional argument prefix when instantiating the commander. sr_hand/arm_commander should both be modified to provide their own prefixes. This would also remove hand/arm specific code, making sr_robot_commander more general.