Tutorial for adjoint optimization of broadband waveguide mode converter with minimum feature size

[oskooi]

Adds a new tutorial for the adjoint solver demonstrating some of its advanced functionality. This is an initial working version and will likely be modified and expanded in future PRs.

Based mainly on results in NanoComp/photonics-opt-testbed#32.

@smartalecH and @mochen4, would you review the text under the new section "Broadband Waveguide Mode Converter with Minimum Feature Size" in Adjoint_Solver.md and make any revisions as necessary?

[codecov-commenter]

Codecov Report

Merging #2365 (d1a98c6) into master (d29eadc) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2365      +/-   ##
==========================================
- Coverage   72.55%   72.51%   -0.05%     
==========================================
  Files          17       17              
  Lines        5156     5148       -8     
==========================================
- Hits         3741     3733       -8     
  Misses       1415     1415              

Impacted Files	Coverage Δ
python/timing_measurements.py	91.66% <0.00%> (-0.84%)	⬇️
python/chunk_balancer.py	88.88% <0.00%> (-0.14%)	⬇️
python/visualization.py	36.58% <0.00%> (-0.10%)	⬇️
python/adjoint/wrapper.py	98.43% <0.00%> (-0.03%)	⬇️
python/simulation.py	76.61% <0.00%> (-0.01%)	⬇️