Add support for SEVIRI Native files without archive header

[sfinkens]

This PR adds support for reading SEVIRI Native files without ASCII archive header. These are provided to users of the European Weather Cloud. The header is only included if data are ordered through the EUMETSAT data center.

• Tests added
• Passes flake8 satpy

[codecov]

Codecov Report

Merging #1612 (f9b8686) into master (3623c03) will increase coverage by 0.00%.
The diff coverage is 95.91%.

@@           Coverage Diff           @@
##           master    #1612   +/-   ##
=======================================
  Coverage   92.38%   92.39%           
=======================================
  Files         254      254           
  Lines       37434    37455   +21     
=======================================
+ Hits        34584    34605   +21     
  Misses       2850     2850           

Flag	Coverage Δ
behaviourtests	4.77% <0.00%> (-0.01%)	⬇️
unittests	92.52% <95.91%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
satpy/readers/seviri_l1b_native.py	79.56% <80.00%> (+0.45%)	⬆️
satpy/readers/seviri_l1b_native_hdr.py	100.00% <100.00%> (ø)
satpy/tests/reader_tests/test_seviri_l1b_native.py	99.44% <100.00%> (+0.01%)	⬆️

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[ameraner]

LGTM, thanks a lot for this, very useful! Just tested it with our archived data, works like a charm :)

[mraspaud]

LGTM

[sfinkens]

Glad to hear, thanks for testing @ameraner !