vgac time units

[BengtRydberg]

This PR deals with #93
and makes that time unit is milliseconds since 1970-01-01 and in vgac files.

• Closes VGAC time unit is in microseconds not milliseconds #93
• Tests added
• Tests passed: Passes pytest level1c4pps
• Passes flake8
• Fully documented

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Details

• 7 of 7 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 80.297%

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Totals
Change from base Build 11367953830:	0.02%
Covered Lines:	1618
Relevant Lines:	1975

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💛 - Coveralls

[ninahakansson]

I tried to add the line:
scn_["scanline_timestamps"].data = scn_["scanline_timestamps"].data.astype('datetime64[ms]'),
just before saving to dataset and then i got milliseconds while still having datatype as int64.
Still some warnings though.

UserWarning: Converting non-nanosecond precision datetime values to nanosecond precision. This behavior can eventually be relaxed in xarray, as it is an artifact from pandas which is now beginning to support non-nanosecond precision values. This warning is caused by passing non-nanosecond np.datetime64 or np.timedelta64 values to the DataArray or Variable constructor; it can be silenced by converting the values to nanosecond precision ahead of time.

[BengtRydberg]

I tried to add the line: scn_["scanline_timestamps"].data = scn_["scanline_timestamps"].data.astype('datetime64[ms]'), just before saving to dataset and then i got milliseconds while still having datatype as int64. Still some warnings though.

UserWarning: Converting non-nanosecond precision datetime values to nanosecond precision. This behavior can eventually be relaxed in xarray, as it is an artifact from pandas which is now beginning to support non-nanosecond precision values. This warning is caused by passing non-nanosecond np.datetime64 or np.timedelta64 values to the DataArray or Variable constructor; it can be silenced by converting the values to nanosecond precision ahead of time.

I verified that your suggestion seems to work. A bit odd why it works, but I added the casting to datetime64/ms and a test for it. Seems like the remaining warnings come from satpy.