Various fixes to waveguide mode converter topology optimization tutorial example

[oskooi]

1. Replace MMA algorithm with CCSA due to Epigraph slack hyperparameter? #2400.
2. Remove unnecessary file context manager when writing Numpy archive file at the end of the optimization.
3. Minor fixes and tweaks to docstrings and function names.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.78%. Comparing base (f29a8c7) to head (8af3a5c).
Report is 26 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2878      +/-   ##
==========================================
- Coverage   73.81%   73.78%   -0.04%     
==========================================
  Files          18       18              
  Lines        5423     5428       +5     
==========================================
+ Hits         4003     4005       +2     
- Misses       1420     1423       +3