POLREF: Current sheet Dual stage motor

[John-Holt-Tessella]

The polref motor uses Dual position stage in SECI we need it to work in IBEX.

The behaviour is:

1. Setting the setpoint to 0 moves the current sheet to the lower limit and then sets the rbv/sp to 0
2. Setting the setpoint to 1 moves the current sheet to the upper limit and then sets the rbv/sp to 1 (it goes via 130, e.g. 0 -> 130 ->1)

This behaviour was retained under IBEX but it didn't know what to do with it.

Acceptance Criteria

• Control a dual position stage
• Document solution

[John-Holt-Tessella]

@John-Holt-Tessella check this is current sheet not monitor

[John-Holt-Tessella]

Fix on beam line just made 0 in beam (with offset) and park be at limit position

[JamesKingWork]

Should this be on the IBEX project board as well?

[John-Holt-Tessella]

Documented solution at https://github.com/ISISComputingGroup/ibex_developers_manual/wiki/Migrating-Galil-motors-from-SECI-to-IBEX#dual-position-stage-axis