Added calibrate command to UdpControl and Client

Added a new AT command, "CALIB", which according to the SDK docs "asks the drone to calibrate the magnetometer (must be flying)."

Tested from a repl.  Note that this causes the drone to yaw 360 degrees.

The command takes a single argument, which is a device number.  Currently the only device the SDK defines is the magnetometer, which is device number 0.