Add ability to 'sleep' arm in static positions

[bl2060]

At the discretion of the commander (user interface or automated director), it should be possible to put the arm/operator into 'sleep' mode. In some strenuous positions, the arm may try to keep an extremely precise position, resulting in overdriving the motor (causing noise, heat, and unnecessary power draw). In positions like aiming at a static point (such as projector or lectern), there should be the option to sleep the operator to prevent these from happening.