EMU: VTI pressure-control cryostat: set pressure PIDs from software

## Where?

As an instrument scientist on EMU I would like to be able to control the pressure PID parameters from software when running the VTI cryostats in automatic pressure-control mode - where IBEX calculates and sends a pressure to the pressure card of a mercury based on the current temperature & setpoint temperature.

Currently the Mercury is set up in auto-PID mode on the pressure card, and manual PID mode on the temperature card.

This ticket is to have a meeting with EMU scientists & cryogenics to fully understand:
- Why this is needed functionality and how they would like it to work
- What the existing PID parameters are (that are being defined by the mercury)
- Whether the PIDs from settemp should set the *temperature card* PIDs, the *pressure card* PIDs, or both
- Whether this wants to be done via `settemp` or whether IBEX needs to support this via something like readascii
- etc

## How?

Arrange a meeting with EMU & cryogenics to discuss the above.

## Acceptance criteria:

- [ ] Meeting had including EMU scientists & cryo group
- [ ] Follow-up tickets generated for implementation

## How to Test
_verbose instructions for reviewer to test changes
(**Add before making a PR**)_