date format: 2011/10/15 05:00:00 or 2011/10/15-05:00:00 or 2011-10-15 05:00:00 or 2011-10-15-05:00:00 or 2011-10-15T05:00:00 or 2011-Oct-15 etc. The time portion is optional.
Returns:
floating point epoch
Note: returns 2000.000 for 2000/01/01 12:00:00 UT as per definition
Examples:
CASA <2>: au.yearFraction()
Out[2]: 2016.1542776323113
CASA <3>: au.yearFraction('2012/01/1 12:00:00')
Out[3]: 2012.0