Add eig_vol argument to EigenModeSource class constructor

[oskooi]

Fixes #2755.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.79%. Comparing base (9cc041e) to head (62ea453).
Report is 40 commits behind head on master.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2767   +/-   ##
=======================================
  Coverage   73.78%   73.79%           
=======================================
  Files          18       18           
  Lines        5421     5423    +2     
=======================================
+ Hits         4000     4002    +2     
  Misses       1421     1421           

Files with missing lines	Coverage Δ
python/source.py	94.08% <100.00%> (+0.03%)	⬆️

[oskooi]

Note: I used this PR along with the one-line change in mpb.cpp in #2285 to verify the accuracy of the adjoint gradients for the EigenmodeCoefficient objective function using the 2d test involving a 1d binary grating described in #2054 (comment). After this PR and #2285 are merged, we may therefore add the 1d diffraction grating as a new unit test for the adjoint solver.

[stevengj]

LGTM.