The current state of the Atomic Clock module. Under normal conditions, from power up, the state sequence should move from unplugged (may be skipped) to warming, ready, ppsShifting, shortTermSteering, averaging (will stay a while in this state) and eventually longTermSteering. In the latter two states, the module is holdover ready. When primary reference (e.g. GPS) is lost, the state should change to holdover, and when holdover is no longer valid state will move to free-running. Normally, 'other' state should not be present.