Fix issue with reading MSG GRIB products from the eumetsat datastore

[ColinDuff]

…etsat datastore and added AES, CTH, MPE and ClearSkyMap GRIB products

Resolves Issue 1477 by adding the EUMETSAT Datastore file pattern

Also further Seviri Grib products were added

A line of code in the reader was also fixed as it didnt seem to be working correctly

Closes #1477
Test script ran and no problems were observed
Passes flake8 satpy

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Codecov Report

Merging #1503 (3ea7001) into master (33c3c8d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1503   +/-   ##
=======================================
  Coverage   91.62%   91.62%           
=======================================
  Files         246      246           
  Lines       36019    36008   -11     
=======================================
- Hits        33001    32993    -8     
+ Misses       3018     3015    -3     

Flag	Coverage Δ
behaviourtests	4.38% <0.00%> (-0.04%)	⬇️
unittests	92.17% <100.00%> (+0.08%)	⬆️

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

Impacted Files	Coverage Δ
satpy/readers/seviri_l2_grib.py	86.73% <100.00%> (ø)
satpy/utils.py	23.41% <0.00%> (-0.69%)	⬇️
satpy/composites/config_loader.py	84.61% <0.00%> (-0.25%)	⬇️
satpy/readers/__init__.py	97.40% <0.00%> (-0.06%)	⬇️
satpy/readers/agri_l1.py	99.15% <0.00%> (-0.03%)	⬇️
satpy/tests/test_scene.py	99.72% <0.00%> (-0.01%)	⬇️
satpy/plugin_base.py	88.00% <0.00%> (ø)
satpy/readers/eps_l1b.py	76.22% <0.00%> (ø)
satpy/composites/viirs.py	83.89% <0.00%> (ø)
satpy/tests/test_utils.py	100.00% <0.00%> (ø)
... and 14 more

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[ColinDuff]

@mraspaud @djhoese
can you have a look at this PR - mainly yaml file updates
does it look ok to you

[djhoese]

Two small comments inline otherwise it looks pretty good. If there are a lot of fields in these files it may be time to try to replace the long set of yaml datasets with the available_datasets method in the reader to dynamically determine what's available from the files provided.

[djhoese]

Thanks for the updates. LGTM

[mraspaud]

LGTM