Fix start_time type in seviri_l1b_nc reader

[BENR0]

Trying to write out geotiff files from data loaded with seviri_l1b_nc
reader gives Error because the reader sets start_time and end_time as
string instead of datetime. This commit fixes this error.

[djhoese]

Thanks for this. Could you also remove the _time_format function that seems to not be used anymore?

[BENR0]

Removed the function.
Something else: I haven't checked if it already is, but if not maybe the documentation on writing own readers should mention that start and end_time attributes should be of type datetime and not string in order to be compatible with the writers?

[coveralls]

Coverage increased (+0.004%) to 77.449% when pulling 8d1a14c on BENR0:fix_start_time_seviri_l1b_nc_reader into 630cc37 on pytroll:master.

[codecov]

Codecov Report

Merging #588 into master will increase coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #588      +/-   ##
==========================================
+ Coverage   77.44%   77.44%   +<.01%     
==========================================
  Files         136      136              
  Lines       19153    19152       -1     
==========================================
  Hits        14833    14833              
+ Misses       4320     4319       -1

Impacted Files	Coverage Δ
satpy/readers/seviri_l1b_nc.py	17.82% <0%> (-0.63%)	⬇️
satpy/multiscene.py	79.12% <0%> (+0.11%)	⬆️

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[djhoese]

@sjoro @ColinDuff I assume one of you worked on this reader? Are these changes ok with you?

[sjoro]

looks good to me!

[ColinDuff]

fine by me