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

[Tom-Willemsen]

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).

[KathrynBaker]

Please note, dead reckoning was not included as it is not used frequently (or at all when the PSUs and readbacks were updated within SECI by myself), and to my knowledge at least the code in the SECI driver is not guaranteed to be correct due to that.