Fix AMI L1b reader incorrectly grouping files

[djhoese]

The satpy.readers.group_files function is using the sector_info filename metadata as one piece of information to sort files by. However, with names like gk2a_ami_le1b_nr013_fd020ge_201909300300.nc where sector_info is fd020ge, you end up with incorrect sorting because it contains resolution information. Compare this to ABI where sector_info might be "F" or "C" or "M1" or "M2" but does not container resolution information.

This PR modifies the filename pattern to limit the "sector_info" to the first two characters of this portion of the filename. This is a guess as currently (as far as I know) there are no other sectors of AMI data available to the public.

@simonrp84 Have you seen other sectors or other types of files?

• Tests added and test suite added to parent suite
• Tests passed
• Passes flake8 satpy

[coveralls]

Coverage increased (+0.02%) to 87.511% when pulling 9d26fb5 on djhoese:bugfix-ami-grouping into fa997a0 on pytroll:master.

[codecov]

Codecov Report

Merging #1048 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1048      +/-   ##
==========================================
+ Coverage   87.49%   87.52%   +0.02%     
==========================================
  Files         188      188              
  Lines       29238    29250      +12     
==========================================
+ Hits        25582    25601      +19     
+ Misses       3656     3649       -7

Impacted Files	Coverage Δ
satpy/tests/test_yaml_reader.py	99.15% <100%> (+0.01%)	⬆️
satpy/tests/reader_tests/test_ami_l1b.py	96.61% <100%> (+1.82%)	⬆️
satpy/readers/yaml_reader.py	94.4% <100%> (ø)	⬆️
satpy/scene.py	89.91% <0%> (+0.17%)	⬆️
satpy/readers/__init__.py	94.56% <0%> (+1.2%)	⬆️

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[mraspaud]

LGTM