Refactor MVIRI dataset access

[sfinkens]

Split DatasetWrapper into

• Dataset Preprocessor: steps that can be done once (e.g. decoding according to CF conventions)
• Dataset Accessor: things that need to be done each time a variable is accessed (e.g. renaming y_* coordinates to y)

Also, test with an actual file. These are internal changes not visible to users.

• Tests added

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.08%. Comparing base (d62b5d2) to head (c7bbfe5).
Report is 116 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2941      +/-   ##
==========================================
- Coverage   96.11%   96.08%   -0.03%     
==========================================
  Files         377      377              
  Lines       55067    55162      +95     
==========================================
+ Hits        52929    53004      +75     
- Misses       2138     2158      +20     

Flag	Coverage Δ
behaviourtests	3.94% <0.00%> (-0.02%)	⬇️
unittests	96.18% <100.00%> (-0.03%)	⬇️

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[coveralls]

Pull Request Test Coverage Report for Build 11975203703

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

• 83 of 83 (100.0%) changed or added relevant lines in 2 files are covered.
• 34 unchanged lines in 12 files lost coverage.
• Overall coverage decreased (-0.008%) to 96.194%

---

Files with Coverage Reduction	New Missed Lines	%
satpy/tests/writer_tests/test_cf.py	1	99.14%
satpy/tests/reader_tests/test_mersi_l1b.py	1	99.71%
satpy/enhancements/init.py	1	99.07%
satpy/tests/reader_tests/test_li_l2_nc.py	1	99.76%
satpy/tests/enhancement_tests/test_enhancements.py	2	99.47%
satpy/tests/utils.py	2	93.16%
satpy/tests/reader_tests/gms/test_gms5_vissr_l1b.py	3	98.67%
satpy/tests/reader_tests/gms/test_gms5_vissr_navigation.py	3	97.18%
satpy/readers/eum_l2_grib.py	3	95.83%
satpy/readers/sar_c_safe.py	5	97.35%

Totals
Change from base Build 11403042925:	-0.008%
Covered Lines:	53248
Relevant Lines:	55355

---

💛 - Coveralls

[sfinkens]

@bkremmli Please also review if you have time

[mraspaud]

A very small comment inline, otherwise LGTM!

[bkremmli]

@bkremmli Please also review if you have time

looks good to me!