ENH: special syntax to allow for referring to PV name in linked variable

[klauer]

This is subject to change and as such I don't want it to be documented just yet (I know, I know...)

Hoping to get this in for a paired PR in lcls-twincat-general shortly (assuming tests pass).

[codecov-io]

Codecov Report

Merging #186 into master will increase coverage by 0.27%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #186      +/-   ##
==========================================
+ Coverage   79.83%   80.11%   +0.27%     
==========================================
  Files          18       18              
  Lines        2237     2253      +16     
==========================================
+ Hits         1786     1805      +19     
+ Misses        451      448       -3

Impacted Files	Coverage Δ
pytmc/record.py	94.69% <100%> (+0.07%)	:arrow_up:
pytmc/parser.py	88.98% <0%> (+0.18%)	:arrow_up:
pytmc/bin/stcmd.py	87.2% <0%> (+0.2%)	:arrow_up:
pytmc/pragmas.py	96.93% <0%> (+0.51%)	:arrow_up:
pytmc/bin/db.py	48.64% <0%> (+1.8%)	:arrow_up:

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[klauer]

Here's a sample of the important records generated to take TEST:LINK and put it in the PLC in MAIN.fbTest. While it uses TEST:EPICSLink* to make this happen, they are not really meant to be a user-facing set of records:

record(ao, "TEST:EPICSLink") {
  field(DESC, "ads:MAIN.fbTest.fPLCInternalValue")
  field(SCAN, ".5 second")
  field(DTYP, "asynFloat64")
  field(DOL, "TEST:LINK CPP MS")
  field(OMSL, "closed_loop")
  field(OUT, "@asyn($(PORT),0,1)ADSPORT=851/MAIN.fbTest.fPLCInternalValue=")
  field(PREC, "3")
  info(autosaveFields_pass0, "PREC VAL")
}

record(longout, "TEST:EPICSLink:LinkSeverity") {
  field(DESC, "ads:MAIN.fbTest.iPLCInternalSeverity")
  field(SCAN, ".5 second")
  field(DTYP, "asynInt32")
  field(DOL, "TEST:EPICSLink.SEVR CPP MS")
  field(OMSL, "closed_loop")
  field(OUT, "@asyn($(PORT),0,1)ADSPORT=851/MAIN.fbTest.iPLCInternalSeverity=")
  info(autosaveFields_pass0, "VAL")
}