Unit test for adjoint solver involving multiple objective functions

[oskooi]

Adds a missing unit test for the adjoint solver involving two separate objective functions for a test problem involving a waveguide mode converter: (1) $R$ and (2) $1-T$, where $R$ is reflectance into the first-order mode of Port 1 and $T$ is transmittance into the second-order mode of Port 2.

The two adjoint simulations (one for each objective function) are set up to run serially but we could modify the setup to have them run in parallel using the meep.divide_parallel_processes feature which is described in Section 5 ("Computational parallelism") of the manuscript.

Also fixes a small bug in the unit test added in #2271 in which the DFT monitor for the reflected fields and source were overlapping (they should be offset from one another).

[codecov-commenter]

Codecov Report

Merging #2294 (b0c1f62) into master (5fa12b4) will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2294      +/-   ##
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+ Coverage   73.23%   73.29%   +0.06%     
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  Files          17       17              
  Lines        4931     4931              
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+ Hits         3611     3614       +3     
+ Misses       1320     1317       -3     

Impacted Files	Coverage Δ
python/adjoint/optimization_problem.py	59.48% <0.00%> (+1.53%)	⬆️