Fix combination of raw metadata (again)

[sfinkens]

In my recent attempt to fix the combination of raw metadata I updated satpy.dataset.metadata.combine_metadata to drop the raw_metadata attribute. This works fine with single-file datasets (e.g. SEVIRI Native), but for multi-file datasets (e.g. SEVIRI HRIT) the attribute is dropped even if only a single dataset has been loaded. This is of course not desired.

This PR enables combine_metadata to compare (nested) dictionaries, so that the removal of the raw_metadata attribute is not necessary anymore. In the end it wasn't as complicated as I thought.

• Tests added

[codecov]

Codecov Report

Merging #1670 (d15bb5b) into main (2a1110e) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1670      +/-   ##
==========================================
+ Coverage   92.66%   92.67%   +0.01%     
==========================================
  Files         258      258              
  Lines       37988    38071      +83     
==========================================
+ Hits        35200    35283      +83     
  Misses       2788     2788              

Flag	Coverage Δ
behaviourtests	4.84% <18.18%> (+0.01%)	⬆️
unittests	92.81% <100.00%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
satpy/dataset/metadata.py	99.07% <100.00%> (+0.38%)	⬆️
satpy/tests/test_dataset.py	100.00% <100.00%> (ø)
satpy/dataset/dataid.py	94.05% <0.00%> (-0.23%)	⬇️
satpy/writers/cf_writer.py	95.17% <0.00%> (-0.02%)	⬇️
satpy/tests/test_multiscene.py	99.46% <0.00%> (+0.04%)	⬆️
satpy/writers/geotiff.py	90.90% <0.00%> (+0.16%)	⬆️
satpy/tests/writer_tests/test_geotiff.py	97.87% <0.00%> (+0.19%)	⬆️
satpy/multiscene.py	91.10% <0.00%> (+0.33%)	⬆️

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[djhoese]

While I'm OK with this, it is only partially because I don't use SEVIRI data on a regular basis. What kind of performance issues can we expect with something like this?

If they are significant, maybe dict comparison should be turned off by default and have a keyword argument to enable it? Then the SEVIRI file handlers could override the base combine_info method and either copy the first raw_metadata dictionary or enable the dict comparison with the keyword argument?

[sfinkens]

For SEVIRI HRIT it increases the runtime of _combine_shared_info by 18%, but it's still only 2 milliseconds in total

[djhoese]

Looks good from what I understand.