add fields to an existing record

["Wolfman, Mark via Tech-talk" <tech-talk at aps.anl.gov>]

Hi, tech-talkers.

Apologies in advance if parts of this don't make sense, I'm not that familiar with EPICS yet.

I'm hoping to add a new region-of-interest field [1] to the MCA records used in an existing IOC for our DXP fluorescence detector. 

Similar to the other ROI fields, I'd like to have ``20bmxmap4b:mca1.R0BH``.

This will be a 'bi' field that doesn't do anything in the IOC, but is used by bluesky/ophyd during staging to determine whether the specific ROI is hinted in bluesky (hence BH for "bluesky hint") [2] and is included in the calculations for summing ROI's across elements. I've added extra records to an IOC before, and so I tried adding this as a record but EPICS complained because my record name included a '.' character.

One solution is to add a record like "20bmxmap4b:mca1_R0BH", but this makes the Ophyd part more complicated since the Ophyd MCA support expects the ROI to have a ``.Rn`` prefix and then tries to append the ``BH`` on the end[3]. Adding a record like ``mca1_R0BH`` means I have to re-write the MCA support in ophyd so I'd prefer to use the ``mca1.RnBH`` PVs if I can.

The full IOC is available on gitlab, though it's presently a work in progress.

Looking forward to hearing any ideas on how to make this work.

Thanks,
Mark

[1] https://epics-modules.github.io/mca/mcaRecord.html#Region-Of-Interest_Fields
[2] https://blueskyproject.io/ophyd/user_v1/reference/signals.html#kind
[3] https://github.com/bluesky/ophyd/blob/0802c77ec973b0ba9f8554d3bcf3a3001def7f66/ophyd/mca.py#L14
[4] https://git.aps.anl.gov/20BM/20bmdxp/-/tree/main/