Muon zero-field controller: implement "dead reckoning" mode

As an instrument scientist using MuSR, I would like to have the option of using "dead reckoning" mode on the zero field controller.

### Acceptance criteria

- Dead reckoning mode is implemented in IBEX driver
- Behaviour is functionally the same as SECI

### Notes

- This is **not** required for the cycle beginning Spring 2021. However, the scientists would like this mode available after the long shutdown.
- See the Labview vi for details about how dead reckoning mode is implemented (we may also need to check our understanding with the scientists).