Fix seviri_l1b_hrit prologue/epilogue readers

[sfinkens]

Didn't accept the extra calibration keyword arguments

• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff

[coveralls]

Coverage increased (+0.002%) to 78.536% when pulling 4070876 on sfinkens:fix-seviri-hrit-ext-calib into 8eea7cf on pytroll:master.

[codecov]

Codecov Report

Merging #660 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #660      +/-   ##
==========================================
+ Coverage   78.53%   78.55%   +0.01%     
==========================================
  Files         138      138              
  Lines       20073    20089      +16     
==========================================
+ Hits        15765    15781      +16     
  Misses       4308     4308

Impacted Files	Coverage Δ
satpy/readers/seviri_l1b_hrit.py	64.36% <100%> (ø)	⬆️
satpy/tests/reader_tests/test_seviri_l1b_hrit.py	97.05% <100%> (+0.39%)	⬆️

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[mraspaud]

LGTM