TSTEP1 read&write fix

[benvb-97]

closes #850

There seems to be an issue with reading/writing TSTEP1 cards in the bdf. When reading a bdf, TSTEP1 instances are added to the model.tstepnls dictionary. This leads to issues further down the line as the code assumes all values in model.tstepnls are TSTEPNL cards. Parts of the code also assume that TSTEP1s are added to the model.tsteps dictionary, leading to conflicts.

The changes to the code now put TSTEP1 cards into the model.tsteps dictionary. I added a small test to verify that TSTEP1s are correctly read and written.

[codecov]

Codecov Report

❌ Patch coverage is 80.95238% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.53%. Comparing base (32be618) to head (90a365a).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pyNastran/bdf/test/test_bdf.py	42.85%	2 Missing and 2 partials ⚠️

❗ There is a different number of reports uploaded between BASE (32be618) and HEAD (90a365a). Click for more details.

HEAD has 27 uploads less than BASE

Flag	BASE (32be618)	HEAD (90a365a)
	28	1

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #851      +/-   ##
==========================================
- Coverage   77.49%   70.53%   -6.97%     
==========================================
  Files         724      724              
  Lines      277557   277563       +6     
  Branches    29192    29189       -3     
==========================================
- Hits       215105   195773   -19332     
- Misses      54731    73812   +19081     
- Partials     7721     7978     +257     

Files with missing lines	Coverage Δ
pyNastran/bdf/bdf.py	63.59% <ø> (-4.91%)	⬇️
pyNastran/bdf/bdf_interface/attributes.py	95.31% <ø> (ø)
pyNastran/bdf/mesh_utils/remove_unused.py	62.22% <ø> (-23.72%)	⬇️
pyNastran/dev/bdf_vectorized3/bdf.py	38.56% <ø> (ø)
pyNastran/op2/test/test_op2_unit_tests.py	90.76% <100.00%> (-3.78%)	⬇️
pyNastran/bdf/test/test_bdf.py	50.17% <42.85%> (-0.31%)	⬇️

... and 133 files with indirect coverage changes

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[SteveDoyle2]

thanks! Yeah, I'd never used SOL 401. It seemed weird TSTEP1 wasn't a TSTEP.