HIFI Cryomag: Alarm when Keithley 2700 timestamp is close to overflow

As an instrument scietist, I want to be able to tell when the K2700 device timestamp is close to overflowing.

When it overflows, meaningful data stops being sent and the IOC stops reporting.

A simple checkc would be to have a PV that minor alarms above a certain value, and increases to a major alarm when it gets even closer to the overflow.

Note that it may also be worthwhile to put a nagios check on this so that developers can let instrument scientists know, in the case that they haven't seen themselves.

Acceptance criteria:

* [ ] New PV to track timestamp, with appropriate alarms
* [ ] Investigate usefulness of nagios check
* [ ] Add nagios check