Update the solar spectrum code to use the included reference spectra

[simonrp84]

At present, the SolarIrradianceSpectrum class requires the user to enter the filename to the solar spectra they wish to use.
However, Pyspectral comes with a data file containing a solar spectra (2000 ASTM Standard Extraterrestrial Spectrum Reference E-490-00).
This PR updates the class to use the included spectra by default (i.e: In the case that the user does not supply their own via the filename argument. I have also updated the __init__ docstring to make clear that this spectra is used by default.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fd32fe9) 89.27% compared to head (65f0378) 89.27%.
Report is 9 commits behind head on main.

❗ Current head 65f0378 differs from pull request most recent head 14cdb4c. Consider uploading reports for the commit 14cdb4c to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #202   +/-   ##
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  Coverage   89.27%   89.27%           
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  Files          23       23           
  Lines        2545     2545           
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  Hits         2272     2272           
  Misses        273      273           

Flag	Coverage Δ
unittests	89.27% <100.00%> (ø)

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[djhoese]

I'm looking at the use of pkg_resources here in my #207 PR and noticed that in a previous PR where I modified MANIFEST.in that this data file is no longer included in the source distribution. I'll need to fix this.

[djhoese]

This PR should update all the files in doc/ that refer to this variable and use it. The code examples should be much simpler and maybe a note could be added to one or more of these docs pages (I see 3 files using it) to mention what specific file is being used.

[djhoese]

Ah nevermind my second to last comment. The files are included!