Fix start_time and end_time being lists in native_msg reader

[ColinDuff]

Start Time and End Time should be returned as an object not as a list. This causes some issues when using SIFT

[coveralls]

Coverage remained the same at 72.356% when pulling 5e7f150 on ColinDuff:develop into 855a4ed on pytroll:master.

[codecov]

Codecov Report

Merging #428 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master     #428   +/-   ##
=======================================
  Coverage   72.35%   72.35%           
=======================================
  Files         128      128           
  Lines       16926    16926           
=======================================
  Hits        12247    12247           
  Misses       4679     4679

Impacted Files	Coverage Δ
satpy/readers/native_msg.py	14.95% <0%> (ø)	⬆️

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[djhoese]

Looks good to me. Maybe @sjoro or @adybbroe can comment on if there was a specific reason this was done originally.

Note to self: I need to cherry pick this to the 0.9 maintenance branch and make a 0.9 release.

[sjoro]

i can't remember any particular reason, but i think somewhere along the line of the development for the reader this might have been necessary? dates back to mpop perhaps? also, the reader has worked so there hasn't been a reason to look at this.

[sjoro]

looks good to me!