Add support for Sentinel-2C

[simonrp84]

This PR adds support for the Sentinel-2C satellite.

In addition, the library used by the S2 RSR reader is outdated and does not work with XLSX files. So I changed to use pandas and openpyxl.

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.25%. Comparing base (af71f1e) to head (72f08bc).
Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #234   +/-   ##
=======================================
  Coverage   90.25%   90.25%           
=======================================
  Files          22       22           
  Lines        2515     2515           
=======================================
  Hits         2270     2270           
  Misses        245      245           

Flag	Coverage Δ
unittests	90.25% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[simonrp84]

pre-commit.ci autofix

[mraspaud]

@adybbroe do you have the possibility to look at this?
Yes, looking at it now, sorry for forgetting!

[adybbroe]

LGTM.
Thanks for adding this!

[adybbroe]

And here is an example plotting the SW spectral responses for all three satellites