BUG: create_datetime_index mislabels data file index when dates are out of order.

[jklenzing]

Describe the bug
pysat.utils.time.create_datetime_index assumes that input files are in order. This is not always the case and can result in files being mislabeled as later years.

To Reproduce

import numpy as np
import pysat
year = np.array([2014, 2014, 2008])
day = np.array([1, 2, 365])
uts = np.array([0, 3600, 2700])
pysat.utils.time.create_datetime_index(year=year, day=day, uts=uts)

Expected behavior
This should produce an index corresponding to the three years. Instead, the final date is labeled as '2014-12-31 00:45:00' instead of '2008-12-31 00:45:00'

Screenshots
This area seems to be the culprit:

pysat/pysat/utils/time.py

Lines 316 to 318 in 6374f84

	# Add in seconds since unix epoch to first day
	uts_del += (dt.datetime(year[0], month[0], 1)
	- dt.datetime(1970, 1, 1)).total_seconds()

Desktop (please complete the following information):

• OS: Mac OS X 10.15.7
• Version python 3.8.11
• Other details about your setup that could be relevant

Additional context
See pysat/pysatCDAAC#24 for context

[jklenzing]

Closing with merge of #907