Add group_keys to seviri_l1b_nc reader yaml

[BENR0]

Adds group_keys tag to seviri_l1b_nc reader yaml

• [x ] Closes group_files function returns only one dictionary #850

[codecov]

Codecov Report

Merging #855 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #855   +/-   ##
=======================================
  Coverage   84.09%   84.09%           
=======================================
  Files         167      167           
  Lines       24541    24541           
=======================================
  Hits        20637    20637           
  Misses       3904     3904

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[coveralls]

Coverage decreased (-0.01%) to 84.076% when pulling 834f5f7 on BENR0:seviri_l1b_nc_group_files into 3cc29bb on pytroll:master.

[mraspaud]

Lgtm