Fix nominal time attributes in SEVIRI HRIT

[sfinkens]

Fix nominal_start/end_time attributes in SEVIRI HRIT, which were tuples due to a trailing comma.

• Tests added

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4877082) 95.88% compared to head (f8937d4) 95.89%.
Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2743   +/-   ##
=======================================
  Coverage   95.88%   95.89%           
=======================================
  Files         371      371           
  Lines       52825    52825           
=======================================
+ Hits        50653    50656    +3     
+ Misses       2172     2169    -3     

Flag	Coverage Δ
behaviourtests	4.15% <33.33%> (ø)
unittests	95.99% <100.00%> (+<0.01%)	⬆️

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[coveralls]

Pull Request Test Coverage Report for Build 7901641731

Details

• 0 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall first build on fix-seviri-nominal-time at 95.973%

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Totals
Change from base Build 7801990864:	96.0%
Covered Lines:	50528
Relevant Lines:	52648

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💛 - Coveralls

[djhoese]

Someone more involved in SEVIRI reader development (including @sfinkens) can merge this. I don't feel comfortable doing it.

[pnuu]

LGTM