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In addition, the library used by the S2 RSR reader is outdated and does not work with XLSX files.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.

• [x] Tests passed: Passes pytest pyspectral
• [x] Passes flake8 pyspectral
• [x] Fully documented

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #234 +/- ## ======================================= Coverage 90.25% 90.25% ======================================= Files 22 22 Lines 2515 2515 ======================================= Hits 2270 2270 Misses 245 245 ``` | [Flag](https://app.codecov.io/gh/pytroll/pyspectral/pull/234/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/pytroll/pyspectral/pull/234/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `90.25% <100.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: 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]

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