add missing FDTD steady state field in doc

[simbilod]

Forgot to export the last part of the notebook to jupytext last time

[codecov]

Codecov Report

Merging #129 (9a473b6) into main (a04647f) will increase coverage by 0.06%.
Report is 9 commits behind head on main.
The diff coverage is 73.17%.

@@            Coverage Diff             @@
##             main     #129      +/-   ##
==========================================
+ Coverage   40.14%   40.20%   +0.06%     
==========================================
  Files         170      170              
  Lines       10529    10550      +21     
==========================================
+ Hits         4227     4242      +15     
- Misses       6302     6308       +6     

Files Changed	Coverage Δ
gplugins/klayout/dataprep/regions.py	44.64% <0.00%> (-1.66%)	⬇️
gplugins/modes/find_modes.py	81.66% <66.66%> (-2.01%)	⬇️
gplugins/modes/find_neff_ng_dw_dh.py	45.45% <100.00%> (-0.82%)	⬇️
gplugins/modes/get_mode_solver_coupler.py	28.00% <100.00%> (ø)
gplugins/modes/get_mode_solver_cross_section.py	43.18% <100.00%> (ø)
gplugins/modes/get_mode_solver_rib.py	75.00% <100.00%> (ø)
gplugins/modes/types.py	33.62% <100.00%> (+2.57%)	⬆️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more