Changes to RCWA

[phoebe-p]

Various changes:

• fix bug in necessary_materials which made all shapes in a layer the same as the last shape that is added (all the dictionaries were just references to one another rather than separate dictionaries)
• add the ability to specify materials for rcwa_structure in terms of n and k (either constant or wavelength-dependent), not as a Solcore material.
• rename rcwa_options to S4_options (this has been done in multiple branches I think...)
• add plotting capabilities to rcwa_structure: plotting the Fourier transform of epsilon, and scanning S4.GetLayerZIntegral and S4.GetFields across x and y points and plotting results.
• rename RCWA_wl method inside rcwa_structure to RCWA_structure_wl; there is already an RCWA_wl function in rcwa.py and this could easily lead to confusion.

[codecov[bot]]

Codecov Report

Merging #6 into devel will decrease coverage by 4.32%. The diff coverage is 20.18%.

@@            Coverage Diff             @@
##            devel       #6      +/-   ##
==========================================
- Coverage   69.68%   65.36%   -4.33%     
==========================================
  Files          19       19              
  Lines        2055     2200     +145     
==========================================
+ Hits         1432     1438       +6     
- Misses        623      762     +139     

Impacted Files	Coverage Δ
rayflare/rigorous_coupled_wave_analysis/rcwa.py	52.18% <20.18%> (-17.29%)	:arrow_down:

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