Add support for SEVIRI Native

[sfinkens]

Add support for SEVIRI L1.5 data in Native format as provided in the European Weather Cloud.

Depends on

• https://github.com/pytroll/satpy/pull/1457
• https://github.com/pytroll/satpy/pull/1468
• https://github.com/pytroll/satpy/pull/1609
  • https://github.com/pytroll/satpy/pull/1612

Also adds two new flags for seviri2pps:

• --use-nominal-time-in-filename to use nominal timestamp for the output filename
• --no-sun-earth-distance-correction to disable sun-earth-distance-correction (needed by CPP)

[sfinkens]

Thanks for reviewing the changes @ninahakansson ! I think we can leave this PR open until the satpy PRs are merged, just in case something changes there.

[sfinkens]

Oops, sorry for the noise

[mraspaud]

All these PR's have been merged and released :)

[codecov-commenter]

Codecov Report

Merging #50 (426693e) into master (015c986) will increase coverage by 1.54%. The diff coverage is 90.38%.

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
+ Coverage   75.11%   76.65%   +1.54%     
==========================================
  Files          21       21              
  Lines        1326     1418      +92     
  Branches      102      110       +8     
==========================================
+ Hits          996     1087      +91     
+ Misses        308      306       -2     
- Partials       22       25       +3     

Flag	Coverage Δ
unittests	76.65% <90.38%> (+1.54%)	:arrow_up:

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

Impacted Files	Coverage Δ
level1c4pps/seviri2pps_lib.py	78.78% <84.12%> (+5.45%)	:arrow_up:
level1c4pps/tests/test_seviri2pps.py	97.41% <100.00%> (+0.46%)	:arrow_up:

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