Reduce rounding error in POD reader adjust clock drift

[carloshorn]

The following lines introduce a larger rounding error than necessary:

pygac/pygac/pod_reader.py

Lines 436 to 437 in 5c59f2b

	self.times = (self.utcs +
	offsets.astype('timedelta64[s]')).astype(datetime.datetime)

Here, self.utcs is an array of type 'datetime64[ms]', and offset is an array of floating point numbers giving seconds, which requires a conversion to a time delta. However, the conversion to 'timedelta64[s]' strips of the fraction of seconds. In order to stay at the same precision level, the conversion to a time delta should be changed to (1000*offsets).astype('timedelta64[ms]').