IOC: Mercury Itc IOC and emulator for a Mercury Controller

[kjwoodsISIS]

As developer, I would like an EPICs based IOC (not lvdcom) and OPI for a Mercury Controller, so that I can control the various types of cryostat & refrigerator on IRIS. I can then also add other cards which are not supported in labview.

Acceptance Criteria

1. I can view and change the PVs which define the state of the Mercury Controller.
2. I can view and change the state of the Mercury Controller via the IBEX GUI.

Notes

1. Check if work from ISISStat issue (LARMOR: ISISStat - drivers #1199) can be re-used.
2. May have different cards from ISISStat case.

[AdrianPotter]

Re-proposed as part of #2840. We don't have time to convert the Mercury cryostat from an LvDCOM to an EPICS IOC before that ticket is needed on GEM but it would be good to make it a native IOC. The number of controls isn't huge and notably we found that Diamond have already written some support code we could potentially reuse:

https://epics.anl.gov/tech-talk/2015/msg00073.php
http://controls.diamond.ac.uk/downloads/support/OxInstCryojet/