HLX503: post-experiment tweaks

[rerpha]

As an instrument scientist using the ITC powered 3he fridge for the first time (this was on WISH) I would like some changes to be made.

Acceptance Criteria

What is the acceptance criteria?
Following issues are fixed:

• It would be better if the threshold for switching between the HE3POTLO and HE3POTHI thermometers was separate to the threshold used for switching between high and low temperature mode. I think we discussed this when we implemented the thermometer switching, but ended up using the same threshold for speed. Unfortunately this didn’t work out during real usage.
• There are a lot of communications drop outs for some reason which causes the reported temperature to drop down to zero. See attached.
• The switching the heater off every time a new setpoint is sent is messing up the PIDs, causing very noticeable temperature drops and hence time delays each time the setpoint is changed (again, see graph).

Extra Information

Why it is needed?

Where required files/links are

How to Test

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

[rerpha]

support: ISISComputingGroup/EPICS-hlx503#7
ioc: ISISComputingGroup/EPICS-ioc#692
GUI (opi): ISISComputingGroup/ibex_gui#1420
Release notes #7174

[rerpha]

We are going to test on the real sorb tomorrow afternoon so will make changes if needed.

[rerpha]

branches patched over to WISH, seems to be working fine and ran overnight with some tests which looked OK according to instrument scientists. Still some issues with a first recondense being temperamental but not introduced by this ticket. To review check db files look OK. IOC tests don't exist on master just yet as I need to sort them out as part of #6240

[rerpha]

PS drop outs were caused by incorrect default stop bit setting which was different to the ITC503 - have modified to match the ITC and that has solved the comms issues!