add docstrings to adjoint solver

[mochen4]

Add docstrings to objective.py and optimization_problem.py

[codecov-commenter]

Codecov Report

Merging #2556 (499c8f8) into master (37516cf) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master    #2556      +/-   ##
==========================================
- Coverage   73.91%   73.90%   -0.01%     
==========================================
  Files          18       18              
  Lines        5294     5293       -1     
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- Hits         3913     3912       -1     
  Misses       1381     1381              

Impacted Files	Coverage Δ
python/adjoint/optimization_problem.py	59.48% <ø> (ø)
python/adjoint/objective.py	92.38% <100.00%> (-0.04%)	⬇️

... and 2 files with indirect coverage changes

[oskooi]

Replace "Attributes:" with "Args:" and separate it with the one-line summary above it with a line break following the style guide: https://google.github.io/styleguide/pyguide.html#s3.8.3-functions-and-methods.

[mochen4]

I thought for classes, it should be attributes instead of args according to https://google.github.io/styleguide/pyguide.html#s3.8.4-comments-in-classes? And that was how Ian wrote the docstrings for EigenmodeCoefficient previously in #1593.
On the other hand, I am confused about whether to put the docstrings in class or __init__. Based on https://stackoverflow.com/questions/37019744/is-there-a-consensus-on-what-should-be-documented-in-the-class-and-init-docs, it seems to that they should be in class based on an earleir version of google style guide.

[oskooi]

Attributes is used for classes and Args for functions.

For the Attributes section of the docstrings for the EigenmodeCoefficient class, we would need to list its public members, almost anything with a self. prefix defined in the constructor excluding self._ which are generally used for private members.

We should also add a separate Args section to the constructor. The argument descriptions in the constructor docstrings should hopefully be different than the attributes description in the class docstring. The attributes description should be a general summary whereas the argument description is less so.

[mochen4]

I added separate Args sections to the constructors. I tried to make the attribute description more concise and general, and argument description more detailed and elaborated. But they still seem a little redundant to me.

[stevengj]

Clarify that gradient is a list (over objective functions) of lists (over design regions) of 2d arrays (design vars x frequencies) of derivatives. If there is only a single objective function, the outer 1-element list is replaced by just that element, and similarly if there is only one design region, while if there is only 1 frequency then the innermost array is squeezed to a 1d array. (So, for example, if you have only a single objective function, a single design region, and a single frequency, then gradient is simply a 1d array of the derivatives.)

[stevengj]

Suggested change

	objective_functions: list of objective functions on objective_arguments
	objective_functions: list of differentiable functions (callable objects) whose arguments are given by objective_arguments