Fix time extraction from filenames in yaml for SEVIRI Native and NetCDF readers

[ameraner]

This PR fixes the start/end/processing time extraction from filenames in the yaml file_patterns for the SEVIRI native and netcdf readers.

• Tests passed

[ameraner]

For the full discussion on this with @djhoese see Slack messages .

[djhoese]

LGTM. Might be nice to have a test for group_files for this, but I think that might need a larger scale set of tests that are out of scope for this PR (ex. testing every readers files with group_files).

[coveralls]

Coverage remained the same at 90.065% when pulling 2117692 on ameraner:fix-native_filename_yaml_pattern into 452f8ad on pytroll:master.

[sjoro]

LGTM

[codecov]

Codecov Report

Merging #1318 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1318   +/-   ##
=======================================
  Coverage   90.06%   90.06%           
=======================================
  Files         220      220           
  Lines       32420    32420           
=======================================
  Hits        29199    29199           
  Misses       3221     3221           

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[sfinkens]

Good job!