Review Julian date/day algorithms

[astrojuanlu]

Module orbit_predictor.predictors.accurate replaces the function jday from sgp4, which uses an algorithm similar to this one:

https://en.wikipedia.org/wiki/Julian_day#Converting_Julian_calendar_date_to_Julian_Day_Number

which in fact comes from Vallado:

https://github.com/brandon-rhodes/python-sgp4/blob/e4ede3c1dc31ba1673a2c3457c5f97dc892f28b2/sgp4/functions.py#L8-L23

And then orbit_predictor.utils contains this alternative, undocumented implementation:

orbit-predictor/orbit_predictor/utils.py

Lines 366 to 374 in 2feced5

	def juliandate(utc_tuple):
	year, month, day, hour, minute, sec = utc_tuple[:6]
	if month <= 2:
	year -= 1
	month += 12
	return (floor(365.25*(year + 4716.0)) + floor(30.6001*(month+1.0)) + 2.0 -
	floor(year / 100.0) + floor(floor(year / 100.0) / 4.0) + day - 1524.5 +
	(hour + minute / 60.0 + sec / 3600.0) / 24.0)