Add PGPLSN card support

[benvb-97]

Hi Steve,

I've added read/write support (for both BDF and OP2 formats) for the PGPLSN card. This is a property card specific to NX, used for generalised plane strain elements in SOL401. I hope the code meets your standards, but I'm more than happy to make any additions, edits, or removals if that would be helpful.

[codecov]

Codecov Report

❌ Patch coverage is 98.37398% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.48%. Comparing base (6c78125) to head (7811fae).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
pyNastran/bdf/cards/properties/shell.py	96.15%	2 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (6c78125) and HEAD (7811fae). Click for more details.

HEAD has 18 uploads less than BASE

Flag	BASE (6c78125)	HEAD (7811fae)
	21	3

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #849      +/-   ##
==========================================
- Coverage   77.50%   70.48%   -7.02%     
==========================================
  Files         724      724              
  Lines      277524   277678     +154     
  Branches    29185    29199      +14     
==========================================
- Hits       215090   195726   -19364     
- Misses      54718    73980   +19262     
- Partials     7716     7972     +256     

Files with missing lines	Coverage Δ
pyNastran/bdf/bdf.py	63.59% <100.00%> (-4.91%)	⬇️
pyNastran/bdf/bdf_interface/add_card.py	64.80% <100.00%> (-24.28%)	⬇️
pyNastran/bdf/bdf_interface/attributes.py	95.31% <ø> (ø)
pyNastran/bdf/cards/test/test_shells.py	98.54% <100.00%> (-0.61%)	⬇️
pyNastran/bdf/mesh_utils/convert.py	68.97% <ø> (-14.56%)	⬇️
pyNastran/bdf/mesh_utils/remove_unused.py	62.50% <100.00%> (-23.45%)	⬇️
pyNastran/op2/tables/geom/ept.py	66.35% <100.00%> (+0.06%)	⬆️
pyNastran/op2/writer/ept_writer.py	89.17% <100.00%> (-4.85%)	⬇️
pyNastran/bdf/cards/properties/shell.py	81.81% <96.15%> (-0.18%)	⬇️

... and 126 files with indirect coverage changes

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[SteveDoyle2]

Awesome!