Fix mersi 2 angles reading

[ninahakansson]

The MERSI-2 reader could no longer read angles data due to that keyword 'calibration' was always expected.

The error message:

python3.6/site-packages/satpy/readers/mersi2_l1b.py", line 95, in get_dataset
    if dataset_id['calibration'] == 'counts':
KeyError: 'calibration'

• Tests added
• Passes flake8 satpy

[mraspaud]

Nice catch! Looks good, I just have one style comment.

[mraspaud]

The more pythonic way to do this is:

Suggested change

	if 'calibration' in dataset_id and dataset_id['calibration'] == 'counts':
	if dataset_id.get('calibration') == 'counts':

IIRC this should work with DataIDs too.

[ninahakansson]

Ok

[codecov]

Codecov Report

Merging #1407 into master will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1407      +/-   ##
==========================================
+ Coverage   90.56%   90.61%   +0.05%     
==========================================
  Files         228      228              
  Lines       33406    34059     +653     
==========================================
+ Hits        30254    30863     +609     
- Misses       3152     3196      +44     

Impacted Files	Coverage Δ
satpy/readers/mersi2_l1b.py	96.51% <100.00%> (ø)
satpy/tests/reader_tests/test_mersi2_l1b.py	99.63% <100.00%> (+<0.01%)	⬆️
satpy/tests/test_composites.py	99.91% <0.00%> (+0.02%)	⬆️
satpy/composites/__init__.py	84.66% <0.00%> (+1.88%)	⬆️

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[coveralls]

Coverage decreased (-0.003%) to 90.562% when pulling d3a473c on ninahakansson:bugfix_mersi2_angles into 12917ab on pytroll:master.

[mraspaud]

LGTM