Fix regular live data crashes on OFFSPEC

[rbauststfc]

Description of work

Summary of work

When live data is run for the OFFSPEC instrument from the ISIS Reflectometry GUI there seem to be regular crashes. In ReflectometryReductionLiveData we were supporting instruments using different Controls systems by looking up different values for OFFSPEC. OFFSPEC have recently switched to IBEX, so we can now look up their values the same way as for other instruments. This reduces the number of errors that are logged and, from testing on IDAaaS, it appears to improve the stability of live data on OFFSPEC.

I am marking this PR as fixing the linked issue as I don't think there are any reproducible problems left on that ticket now.

Fixes #35400.

To test:

I have tested with Conda packages on IDAaaS because checking the impact on stability requires running it for some time while the beam is on. I ran live data for OFFSPEC in the Nightly alongside running it from the Conda packages and found that in just under 24 hours the Nightly version crashed twice while the Conda packages didn't crash at all. I wouldn't suggest that this be repeated when testing this PR, but the following will ensure that live data is still working OK. I can't seem to run live data on my machine when not plugged into the network on site so I can give installation details for the Conda packages, or share an IDAaaS workspace, if anyone needs to test there instead.

1. Open the ISIS Reflectometry interface.
2. On the Runs tab, change the instrument to OFFSPEC.
3. In the Live Data section on this tab, click the Start monitor button.
4. Check that in the log messages pane you get something like:

THETA = ...
S1VG = ...
S2VG = ...

Rather than the below errors, which you would have seen in the past when trying to read the s1v and s2v values for OFFSPEC:

Error in execution of algorithm GetLiveInstrumentValue:
Error reading EPICS PV "IN:OFFSPEC:CS:SB:s1vgap":

5. Repeat the above for INTER to check that live data is still working OK for other instruments.

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[cailafinn]

LGTM