Use spectral wavelength for relative "band"

[karllark]

When one has optical spectroscopy, it is better to use 0.55 micron as the relative "band" for the elx measurement. This PR enables using a spectral wavelength instead of a photometric band.

[codecov]

Codecov Report

Attention: Patch coverage is 54.83871% with 14 lines in your changes missing coverage. Please review.

Project coverage is 39.65%. Comparing base (e79245f) to head (bbd0b84).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
measure_extinction/extdata.py	54.83%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #125      +/-   ##
==========================================
- Coverage   39.67%   39.65%   -0.03%     
==========================================
  Files          22       22              
  Lines        3017     3039      +22     
==========================================
+ Hits         1197     1205       +8     
- Misses       1820     1834      +14     

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