Issue #502 - Fix broken leapsecond file generation

[MJJoyce]

Update leapsecond file generation with new leapsecond data retrieval location
and update current leapsecond file.

Processed leapsecond data was evaluated against an "up-to-date" file generated
using data pulled from the old location. For comparison, data from the old file
is on top, and new data on bottom. Note the changes in the valid date.

>>> pp.pprint(dmc.LeapSeconds._data)
{   'leapseconds': [   (datetime.datetime(1981, 7, 1, 0, 0), 1),
                       (datetime.datetime(1982, 7, 1, 0, 0), 2),
                       (datetime.datetime(1983, 7, 1, 0, 0), 3),
                       (datetime.datetime(1985, 7, 1, 0, 0), 4),
                       (datetime.datetime(1988, 1, 1, 0, 0), 5),
                       (datetime.datetime(1990, 1, 1, 0, 0), 6),
                       (datetime.datetime(1991, 1, 1, 0, 0), 7),
                       (datetime.datetime(1992, 7, 1, 0, 0), 8),
                       (datetime.datetime(1993, 7, 1, 0, 0), 9),
                       (datetime.datetime(1994, 7, 1, 0, 0), 10),
                       (datetime.datetime(1996, 1, 1, 0, 0), 11),
                       (datetime.datetime(1997, 7, 1, 0, 0), 12),
                       (datetime.datetime(1999, 1, 1, 0, 0), 13),
                       (datetime.datetime(2006, 1, 1, 0, 0), 14),
                       (datetime.datetime(2009, 1, 1, 0, 0), 15),
                       (datetime.datetime(2012, 7, 1, 0, 0), 16),
                       (datetime.datetime(2015, 7, 1, 0, 0), 17),
                       (datetime.datetime(2017, 1, 1, 0, 0), 18)],
    'valid': datetime.datetime(2023, 6, 28, 0, 0)}

----

>>> pp.pprint(dmc.LeapSeconds._data)
{   'leapseconds': [   (datetime.datetime(1981, 7, 1, 0, 0), 1),
                       (datetime.datetime(1982, 7, 1, 0, 0), 2),
                       (datetime.datetime(1983, 7, 1, 0, 0), 3),
                       (datetime.datetime(1985, 7, 1, 0, 0), 4),
                       (datetime.datetime(1988, 1, 1, 0, 0), 5),
                       (datetime.datetime(1990, 1, 1, 0, 0), 6),
                       (datetime.datetime(1991, 1, 1, 0, 0), 7),
                       (datetime.datetime(1992, 7, 1, 0, 0), 8),
                       (datetime.datetime(1993, 7, 1, 0, 0), 9),
                       (datetime.datetime(1994, 7, 1, 0, 0), 10),
                       (datetime.datetime(1996, 1, 1, 0, 0), 11),
                       (datetime.datetime(1997, 7, 1, 0, 0), 12),
                       (datetime.datetime(1999, 1, 1, 0, 0), 13),
                       (datetime.datetime(2006, 1, 1, 0, 0), 14),
                       (datetime.datetime(2009, 1, 1, 0, 0), 15),
                       (datetime.datetime(2012, 7, 1, 0, 0), 16),
                       (datetime.datetime(2015, 7, 1, 0, 0), 17),
                       (datetime.datetime(2017, 1, 1, 0, 0), 18)],
    'valid': datetime.datetime(2024, 6, 28, 0, 0)}

Resolve #502