Add converter for Electro-L N2 MSU/GS spectral response functions

[simonrp84]

This adds the spectral response functions for the Electro-L N2 satellite's MSU-GS sensor. Unfortunately it's only for the N2 sat as I can't find RSRs for the N1 or N3 sats, but will update the code in future if I can get them.

I also added a section to pyspectral.yaml but commented it out and updated the readme. In addition to adding Electro-L to the readme I also took the liberty of changing @adybbroe as author to "Pytroll developers" as I think quite a few people have worked on this. Hope that's OK!

• Tests passed: Passes pytest pyspectral
• Passes flake8 pyspectral
• Fully documented

[codecov]

Codecov Report

Merging #144 (9983f46) into main (247676b) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #144   +/-   ##
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  Coverage   84.18%   84.18%           
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  Files          23       23           
  Lines        2454     2454           
=======================================
  Hits         2066     2066           
  Misses        388      388           

Flag	Coverage Δ
unittests	84.18% <100.00%> (ø)

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Impacted Files	Coverage Δ
pyspectral/utils.py	65.28% <100.00%> (ø)

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[adybbroe]

Looks fine. Just fix the small cut'n'paste thingy

I tested it and this gives me a plot of the responses for the three short wave channels:
python bin/composite_rsr_plot.py -p Electro-L-N2 -s msu-gs --range 0.4 0.9

[adybbroe]

@simonrp84
I will soon add the hdf5 file to zenodo.org and then we can update the URL in utils.py. And then we can merge...

[adybbroe]

As I said in your other PR #145 I have updated the RSR on zenodo.org, so you can update the URL and version in utils.py now.