TTI PL303QMD-P

As an electronics technician, I would like IBEX to support a `TTI PL303QMD-P` power supply.

This is a dual-channel version of the TTI PL-P series of power supplies. Our IOC should be extended to allow additional channels. In our existing IOC we already have an `ADDRESS` macro which is the internal comms address, this ticket it to add `ADDRESS2`..`ADDRESS` macros which conditionally load extra channels. Some refactoring of db/`st.cmd`/emulator will also be required, but the actual commands to the device should be the same as currently.

> The other one is PL303QMD-P (Dual channel PL303-P). It uses single RS232 port for both outputs. This will mean we do not need use two IOCs to run supplies for experiments.
>
> There is no rush at all!

### Acceptance criteria
- [ ] We can talk to both channels on a `TTI PL303QMD-P` power supply using the existing `TTIPLP` ioc.
- [ ] OPI updated to show both channels if both are configured
- [ ] By default the IOC still only loads a single channel (remaining compatible with existing IOC)
- [ ] An upgrade script has been created if any existing macro names or PV names have been changed.
- [ ] Consider looking at the [TTI website](https://www.aimtti.com/product-category/dc-power-supplies/aim-plseries) and allowing enough channels for the biggest power supply they sell - in case we later get a 3-channel one

Start with the existing TTIPLP IOC and its ADDRESS macro, then inspect the related db, st.cmd, emulator, and OPI components. Trace how one channel is loaded and how the existing device commands are used. Done means both channels work, single-channel configurations remain compatible, the OPI shows configured channels, and an upgrade script exists if names change.

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