Sensors: create method for registering the gyro rate

This s used by the individual drivers to register
their max rate and later the sanity check code for
verifying the settings.
SanityCheck: check the stabilization rates

Only allow rates that are within 85% of what the gyro can track are
permitted.
SanityCheck: monitor for changes in stab settings
SanityCheck: add a new message for unsafe rates
Merge pull request #1013 from peabody124/safe_sensors_rate

Safe sensors rate