Muon Zerofield System: Add check to Setpoint Readback from PSUs

[davidkeymer]

As a developer, I would like the Zerofield IOC to make sure that it doesn't react to any invalid setpoints read from a power supply.

Acceptance Criterion

• A 'sanity' check is present in the Zerofield code to ignore any invalid PSU setpoint readbacks

Extra Information

During zerofield testing on MuSR, occasional 'spikes' were seen on the X-field. These were caused by an invalid setpoint being read from its associated PSU (Kepco), a correction being added, and then sent back to the PSU as a new setpoint. This was outside the physical range of the device, which defaulted to maximum output which then subsequently saturated the magnetometer.

Related to #7716

How to Test

verbose instructions for reviewer to test changes
(Add before making a PR)

[FreddieAkeroyd]

This may also be relevant #7719

[KathrynBaker]

This needs to go into the same location as some other checks that values are sensible.

[esmith1729]

PRs:

• Added state for psu sp rbv out of limit

Added a new state "ZF_PSU_SP_RBV_OUT_OF_RANGE" and included checks for this out of range behaviour in the zero_field.st file and the corresponding .db for zerofield control.

• Created test in zfcntrl.py test_WHEN_psu_sp_rbv_is_out_of_range_THEN_status_is_psu_sp_rbv_out_of_range

• Covers 6 different cases: above and below limits for x, y, and z
• sets limits to 3 and -3
• simulated rbv output of the psu at 10 or -10, depending on the case
• this triggers the state "PSU is out of range"
• Also added some code into _setUp to set all CA objects for x, y, and z's PSUs to 0, before assigning them any values (the test was failing in the setup phase because of this).

Other changes

• Release Notes

• Updated Zero field controller Wiki

[esmith1729]

ISISComputingGroup/EPICS-IOC_Test_Framework#598

[esmith1729]

#7812