Add flux member function for class dft_near2far in C++ and DftNear2Far in Python

[oskooi]

Fixes #2665.

Requires a (working) SWIG wrapper and a unit test.

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Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 73.81%. Comparing base (e908ef8) to head (1cf5550).
⚠️ Report is 188 commits behind head on master.

Files with missing lines	Patch %	Lines
python/simulation.py	50.00%	1 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2670      +/-   ##
==========================================
- Coverage   73.81%   73.81%   -0.01%     
==========================================
  Files          18       18              
  Lines        5294     5296       +2     
==========================================
+ Hits         3908     3909       +1     
- Misses       1386     1387       +1     

Files with missing lines	Coverage Δ
python/simulation.py	77.04% <50.00%> (-0.03%)	⬇️

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[oskooi]

Turns out this feature is more challenging to implement than I originally thought because add_near2far does not center its DFT fields on the Yee-grid voxels unlike add_flux (see #2538 (comment) for more details). This is why the Poynting flux calculation in add_flux is trivial.