combine_metadata only supports the average of time attrs

[zxdawn]

Describe the bug
Currently, combine_metadata will calculate the mean value of attrs related to time.
But, start_time should be the minimum and end_time should be the maximum.

Expected behavior
MultiScene:

start_time 2019-07-25 05:15:10
end_time 2019-07-25 05:34:49

Actual results
Scene_1:

start_time 2019-07-25 05:15:10
end_time 2019-07-25 05:24:49

Scene_2:

start_time 2019-07-25 05:25:10
end_time 2019-07-25 05:34:49

MultiScene:

start_time 2019-07-25 05:20:10
end_time 2019-07-25 05:29:49

[djhoese]

I really thought we were already doing that, but you're right of course. Here's the PR where I added averaging #473. A fix for this could probably remove the python 2 compatibility stuff too.

[zxdawn]

@djhoese So, by setting average_times=False will lead to the minimum as start_time and maximum as end_time? If so, why not supporting average_times in timeseries?

satpy/satpy/multiscene.py

Line 67 in 65cd83f

res.attrs = combine_metadata(*[x.attrs for x in expanded_ds])

[djhoese]

I'm not sure what you are saying. The combine_metadata function needs to be updated to support min/max'ing start/end_time. I'm not sure what the right way is to handle it in the timeseries function.