Add logic for making signals on-the-fly, sans p4p -vv call

[rosesyrett]

Resolves #25

For this to work, I need to be able to query the epics signal via pv access to get the type of the signal, to be used in the signal_factory (i.e epics_signal_ ... function). Two things need to happen for this to work:

1. The .db file being used (i.e. the one currently in tests/db/panda.db) needs to have enough information about the signal types,
2. I need a way of doing 'pvget -vv ${IOC:PV}' via python's p4p library. I am struggling to see how to do that.
   edit: after discussion it seems like we can just use 'None' and the backend will guess.

[rosesyrett]

@coretl this is almost done, except in my tests the db used does not contain an "active" bool signal in the PVI. Could you just modify this MR to include it? Thanks

[coretl]

Try this:

record(bi, "$(IOC_NAME=PANDAQSRV):PCAP:ACTIVE")
{
    field(ZNAM,  "0")
    field(ONAM,  "1")
    field(PINI, "YES")
    info(Q:group, {
        "$(IOC_NAME=PANDAQSRV):PCAP:PVI": {
            "pvi.active.r": {
                "+channel": "NAME",
                "+type": "plain"
            }
        }
    })
}

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.55% 🎉

Comparison is base (23d4c46) 98.90% compared to head (feb26da) 99.45%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #31      +/-   ##
==========================================
+ Coverage   98.90%   99.45%   +0.55%     
==========================================
  Files           5        5              
  Lines         364      365       +1     
==========================================
+ Hits          360      363       +3     
+ Misses          4        2       -2     

Files Changed	Coverage Δ
src/ophyd_epics_devices/panda.py	99.34% <100.00%> (+1.32%)	⬆️

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[olliesilvester]

Looks good - and works with a real PandA too