Add support for MERSI-1 on FY-3A/B/C

[yukaribbba]

• Tests added
• Fully documented
• RSR for pyspectral

Datasets for test:
https://drive.google.com/file/d/1cCoC14AioC2l34jFfxa19F88k5NXuqpi/view?usp=sharing

1. FY-3A:
   

2. FY_3B:
   

3. FY_3C:
   

[yukaribbba]

pre-commit.ci autofix

[yukaribbba]

@djhoese One of the CI test keeps failing on macos-latest, 3.9, false. I don't know what's going on.

=========================== short test summary info ============================
FAILED satpy/tests/reader_tests/test_sar_c_safe.py::TestSAFEGRD::test_read_lon_lats - AssertionError: 
Not equal to tolerance rtol=1e-07, atol=0

Mismatched elements: 3 / 256 (1.17%)
Max absolute difference: 4.98866104e-09
Max relative difference: 2.33333333
 x: array([[ 2.081090e-16,  5.91[666](https://github.com/pytroll/satpy/actions/runs/8872941895/job/24358006466?pr=2795#step:9:667)7e-01,  9.097222e-01,  1.000000e+00,
         9.083333e-01,  6.805556e-01,  3.625000e-01, -1.110223e-16,
        -3.611111e-01, -6.750000e-01, -8.958333e-01, -9.777778e-01,...
 y: array([[ 3.794929e-16,  5.916[667](https://github.com/pytroll/satpy/actions/runs/8872941895/job/24358006466?pr=2795#step:9:668)e-01,  9.097222e-01,  1.000000e+00,
         9.083333e-01,  6.805556e-01,  3.625000e-01,  8.326673e-17,
        -3.611111e-01, -6.750000e-01, -8.958333e-01, -9.777778e-01,...
= 1 failed, 2541 passed, 4 skipped, 5 xfailed, 1 xpassed, 443 warnings in 352.85s (0:05:52) =

[djhoese]

Yes. This is a known problem with a scipy update causing a test failure. It is being looked at.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.94%. Comparing base (3b9c04e) to head (f48d740).
Report is 53 commits behind head on main.

❗ Current head f48d740 differs from pull request most recent head d04bd36

Please upload reports for the commit d04bd36 to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2795      +/-   ##
==========================================
- Coverage   95.95%   95.94%   -0.01%     
==========================================
  Files         379      379              
  Lines       53888    53764     -124     
==========================================
- Hits        51708    51585     -123     
+ Misses       2180     2179       -1     

Flag	Coverage Δ
behaviourtests	4.10% <0.00%> (+<0.01%)	⬆️
unittests	96.04% <100.00%> (-0.01%)	⬇️

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

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[djhoese]

I know this is still a draft, but I'm curious if there are actual differences between the datasets for the 3 satellite instruments. If they're the same channels then maybe the 3 readers could be combined into a single "mersi_l1b" reader?

[yukaribbba]

Well it's kinda annoying here that there are no genuine differences inside the datasets but they are still organized in different forms. So I think merging them into one could be a little bit messy for the users...

[djhoese]

Yeah MERSI and AGRI were the two instruments I think of with designs like that (new version of the instrument, but completely different numbering/ordering of channels).

[yukaribbba]

pre-commit.ci autofix

[yukaribbba]

pre-commit.ci autofix

[yukaribbba]

@djhoese Some of the CI failed again. But at least the tests passed.

[djhoese]

I restarted the CI. Very odd error from the test coverage reporting tool so let's see if it fixes itself...

[yukaribbba]

Failed again....

[djhoese]

Looks like a major release of the coveralls python package causing issues: https://pypi.org/project/coveralls/

I have appointments half of today, but I'll try debugging later.

[coveralls]

Pull Request Test Coverage Report for Build 9369822041

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 258 of 258 (100.0%) changed or added relevant lines in 2 files are covered.
• 570 unchanged lines in 59 files lost coverage.
• Overall coverage decreased (-0.009%) to 96.023%

---

Files with Coverage Reduction	New Missed Lines	%
satpy/readers/glm_l2.py	1	98.55%
satpy/readers/ascat_l2_soilmoisture_bufr.py	1	97.44%
satpy/tests/multiscene_tests/test_save_animation.py	1	99.1%
satpy/tests/reader_tests/_li_test_utils.py	1	99.26%
satpy/dataset/metadata.py	1	99.26%
satpy/tests/reader_tests/test_li_l2_nc.py	1	99.76%
satpy/tests/reader_tests/test_ascat_l2_soilmoisture_bufr.py	1	97.59%
satpy/readers/mws_l1b.py	1	98.52%
satpy/tests/reader_tests/test_avhrr_l1b_gaclac.py	1	99.66%
satpy/tests/reader_tests/test_ahi_hsd.py	1	99.67%

Totals
Change from base Build 9157044280:	-0.009%
Covered Lines:	51524
Relevant Lines:	53658

---

💛 - Coveralls

[mraspaud]

LGTM, thanks a lot for refactoring the tests! Just one comment inline, but after that I can merge

[mraspaud]

LGTM