Dock should produce buoy and forcefield codes separately.

[shamlian]

The Create 3 dock does not produce op code 173 when the robot is within all three zones (2 buoys + forcefield) as older docks used to do. Instead, it cycles between buoy and forcefield codes. A code should be generated every ~80 ms or so. (This means that if the robot is in only a buoy zone or only a forcefield zone, the code will update only once every ~160 ms.)

See discussion in https://github.com/iRobotEducation/create3_sim/pull/66

[justinIRBT]

We decided to alternate in the existing 62Hz thread if the robot sees both forcefield and buoys, so the effective rate will by 31Hz if it sees both and 62Hz if it only sees 1