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     
==========================================
- 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%)	:arrow_down:

... and 2 files with indirect coverage changes