Replace pcaspy with p4p

[anderslindho]

For the EPICS adapter, Lewis uses pcaspy (which I believe might use legacy asyncio modules). The handling of this dependency also seems a bit odd.

We should instead use the more modern p4p, built on the reimplementation of the PVA stack.

[Tom-Willemsen]

To be honest, I'd forgotten we even shipped an [epics] optional dependency! This functionality is not used at ISIS, and a brief search on github doesn't reveal any downstream repositories depending on lewis[epics]. I'm unsure if it's ever been used other than by ESS.

My first instinct would be to prefer adding a new epicspva optional dependency rather than reusing the epics dependency group for pva servers. This would be comparable to approaches in fastcs or ophyd-async which both also have this pattern of optionally supporting both EPICS CA and PVA.

[anderslindho]

I would be in favour of using a more specific name (e.g. pva and ca), but I think the pcaspy usage should be removed regardless.

As I mentioned in the original report, I think the pcaspy dependency also may relevant for #320.

[Tom-Willemsen]

Ok - I guess I am just being cautious as I don't know for sure whether there are any users are using the existing EPICS CA support.

From a github search, I can only find the following usages:

• https://github.com/michele-brambilla/estiasim/blob/96f9469524ae34559307f1a25810f00c067662df/lewissim/selene_motor/__init__.py#L4 - doesn't look very active.
• https://github.com/michele-brambilla/estiasim/blob/96f9469524ae34559307f1a25810f00c067662df/lewissim/spss7/__init__.py#L3 - same repo as above

Given that the only usages are those above (both seem to be related to the ESS Estia beamline), and the ESSChopper built into lewis, I think I am ok with the pcaspy-based EPICSInterface being removed if neither are needed at ESS any more.

[anderslindho]

6 years ago. I think I remember that name - that they worked at PSI - but I don't know if they work with ESS anymore. I can't find their profile on our intranet (but I do find an account on our Slack instance). That was around the time that Lewis was being developed, IIRC, so maybe just early testing.

I am 99% sure that there's no usage at ESS that I do not know of, because most people who work here nowadays don't even know about Lewis.