Mercury heliox: change modes directly

As a cryogenic support person, I would like the ability to change the heliox mode between "Low Temp", "High Temp" and "Regenerate" directly, rather than relying on the Heliox's built-in regeneration routine (which is triggered when sending TSet=0).

### Acceptance criteria:
- Can change modes directly via the OPI

### Notes
- This will require testing with real hardware
- Beware of heaters getting stuck on/off (see https://github.com/ISISComputingGroup/ControlsWork/issues/87 for details) when changing states. We may need to write a check & correct routine for this.
- This is currently a "nice-to-have" for cryogenics group - not urgent.
- The need for this option arises when the outer cryostat is running a bit warmer than ideal. In this case, the automatic regeneration routine will never finish (see https://github.com/ISISComputingGroup/ibex_developers_manual/wiki/Mercury-Heliox#regeneration-starts-but-never-finishes). Cryogenics group would like to be able to manually kick it out of regeneration mode and back into low temp mode in this scenario.