Report date_dt as fractional days, not full days between

[jhkennedy]

Current implementation will round-down the date_dt attribute to the number of full days between acquisitions.

For an extreme example, start with parsing the acquisition times, and getting the datetime.timedelta

import numpy as np
from datetime import datetime

>>> d0 = datetime.strptime('2020-06-26T20:48:33Z', '%Y-%m-%dT%H:%M:%SZ')
... d1 = datetime.strptime('2020-07-01T20:48:30Z', '%Y-%m-%dT%H:%M:%SZ')
...
... dt = d1 - d0
>>> dt
datetime.timedelta(days=4, seconds=86397)

you can see datetime thinks of this as "four days plus some seconds". So that means, when the current date_dt is calculated, you get:

>>> date_dt = np.float64(dt.days)
>>> date_dt
4.0

This switches to the actual fractional days, like:

>>> dt.total_seconds() / timedelta(days=1).total_seconds()
4.999965277777778

[leiyangleon]

LGTM