Update orbit coefficient docstrings in seviri_l1b_hrit

[sfinkens]

A while ago we were wondering how there can be gaps and overlaps in the list of orbit coefficients in the HRIT header. Now I got a reply from the EUMETSAT Help Desk:

A manoeuvre is a discontinuity in the orbit parameters. The flight dynamic algorithms are not made to interpolate over the time-span of the manoeuvre; hence we have elements describing the orbit before a manoeuvre and a new set of elements describing the orbit after the manoeuvre.
The flight dynamic products are created so that there is an intentional gap at the time of the manoeuvre. In your example that’s between 22:15 and 22:30. This is achieved by setting the validity-end-time of the last pre-manoeuvre elements to hh:00, hh:15, hh:30 or hh:45 before the manoeuvre. Each set of elements spans six hours. In your example the validity-end-time was set to 22:15 and hence the validity-start-time is 16:15. Now, the overlap is not of an issue as both sets of elements describe the same pre-manoeuvre orbit (with negligible variations).
This mechanism is in place for all MSG satellites since the beginning and still is used like this.

I updated the docstrings accordingly.

• Tests passed
• Passes flake8 satpy
• Fully documented

[coveralls]

Coverage increased (+0.004%) to 85.366% when pulling ac32121 on sfinkens:doc-seviri-hrit-orbit-coefs into 24dd827 on pytroll:master.

[codecov]

Codecov Report

Merging #916 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #916   +/-   ##
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  Coverage   85.36%   85.36%           
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  Files         174      174           
  Lines       25912    25912           
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  Hits        22119    22119           
  Misses       3793     3793

Impacted Files	Coverage Δ
satpy/readers/seviri_l1b_hrit.py	92.3% <ø> (ø)	⬆️

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[mraspaud]

LGTM