Move PvaEpicsGUI to pva/gui.py

[GDYendell]

Closes #254

[codecov]

Codecov Report

❌ Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.38%. Comparing base (2895e1f) to head (5cdb4ec).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/fastcs/transport/epics/pva/gui.py	96.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #274      +/-   ##
==========================================
+ Coverage   89.36%   89.38%   +0.01%     
==========================================
  Files          45       46       +1     
  Lines        2276     2280       +4     
==========================================
+ Hits         2034     2038       +4     
  Misses        242      242              

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