Add reader for L1C data from the MSI instrument aboard EarthCARE

[simonrp84]

This PR adds support for Level 1C (regridded) data produced by the MultiSpectral Imager (MSI) aboard the ESA/JAXA EarthCARE mission.

Currently, the pre-launch test data is supported and it's likely that some small changes will be required once the satellite is operational. For now, all existing MSI data works with the reader.

IMPORTANT NOTE: Both the Sentinel-2 and EarthCARE missions have sensors named 'MSI'. Therefore, I've also made some small changes to the S2/MSI reader in this PR in order to delineate between the two mission sensors.

• [x] Tests added
• [x] Fully documented

[codecov[bot]]

Codecov Report

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

Project coverage is 95.96%. Comparing base (3b9c04e) to head (00d5144). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2803 +/- ## ========================================== + Coverage 95.95% 95.96% +0.01% ========================================== Files 379 381 +2 Lines 53888 54026 +138 ========================================== + Hits 51708 51846 +138 Misses 2180 2180 ``` | [Flag](https://app.codecov.io/gh/pytroll/satpy/pull/2803/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | Coverage Δ | | |---|---|---| | [behaviourtests](https://app.codecov.io/gh/pytroll/satpy/pull/2803/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `4.08% <0.00%> (-0.02%)` | :arrow_down: | | [unittests](https://app.codecov.io/gh/pytroll/satpy/pull/2803/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `96.06% <100.00%> (+0.01%)` | :arrow_up: | 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.

[coveralls]

Pull Request Test Coverage Report for Build 9191585206

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 96.043%

---

Totals
Change from base Build 9157044280:	0.01%
Covered Lines:	51718
Relevant Lines:	53849

---

💛 - Coveralls

[yukaribbba]

If you change the sensor name you may also need to make the relative changes on pyspectral/zenodo rsr files. Otherwise there'll be an inconsistent warning.