Update GRIB reader for greater flexibility.

[simonrp84]

Currently, the grib reader expects certain keys to be present in the grib file, for example:

ds_info.update({
            'filename': self.filename,
            'shortName': msg['shortName'],
            'long_name': msg['name'],
            'pressureUnits': msg['pressureUnits'],
            'typeOfLevel': msg['typeOfLevel'],
            'standard_name': msg['cfName'],
            'units': msg['units'],
            'modelName': msg['modelName'],
            'model_time': model_time,
            'centreDescription': center_description,
            'valid_min': msg['minimum'],
            'valid_max': msg['maximum'],
            'start_time': start_time,
            'end_time': end_time,
            'sensor': msg['modelName'],
            # National Weather Prediction
            'platform_name': 'unknown',
        })

For some grib files, such as those from the ERA5 and CAMS datasets, not all keys are present. modelName is not present in ERA5 single level datasets.

This PR adds some flexibility to the grib reader and will now set missing keys to unknown in ds_info rather than raising an error.
I have only tested this with ECMWF/ERA/GFS data but it seems to work well with those.

• Tests passed
• Passes flake8 satpy

[codecov]

Codecov Report

Merging #1586 (f071cb9) into master (f248972) will decrease coverage by 0.18%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1586      +/-   ##
==========================================
- Coverage   92.84%   92.66%   -0.19%     
==========================================
  Files         251      253       +2     
  Lines       37193    37199       +6     
==========================================
- Hits        34532    34470      -62     
- Misses       2661     2729      +68     

Flag	Coverage Δ
behaviourtests	4.78% <0.00%> (+0.30%)	⬆️
unittests	92.80% <100.00%> (-0.19%)	⬇️

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

Impacted Files	Coverage Δ
satpy/readers/grib.py	84.61% <100.00%> (+2.12%)	⬆️
satpy/tests/reader_tests/test_grib.py	97.79% <100.00%> (+0.38%)	⬆️
satpy/tests/utils.py	92.61% <0.00%> (-5.09%)	⬇️
satpy/readers/__init__.py	96.99% <0.00%> (-0.43%)	⬇️
satpy/tests/test_scene.py	99.43% <0.00%> (-0.30%)	⬇️
satpy/tests/reader_tests/test_mirs.py	100.00% <0.00%> (ø)
satpy/readers/mirs.py	71.93% <0.00%> (ø)
satpy/tests/writer_tests/test_cf.py	99.69% <0.00%> (+<0.01%)	⬆️
satpy/tests/test_dependency_tree.py	98.24% <0.00%> (+0.20%)	⬆️
satpy/dataset/dataid.py	94.27% <0.00%> (+0.22%)	⬆️
... and 6 more

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[mraspaud]

LGTM

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[djhoese]

Thanks for the fixes. Will merge when tests pass. We should make sure all the other PRs are closed when it is merged.