Fix failure to convert Time objects with format='unix' or 'cxcsec'

The original TimeFromEpoch (base class for 'cxcsec' and 'unix' formats)
required that the scale be TAI.  This caused failures when trying
to convert to a different scale or initializing a new Time from such
a TimeFromEpoch format object.

    t = Time(100.0, format='unix')
    t2 = t.utc  # crash
    t2 = Time(t, format='iso')  # crash

New implementation does on-the-fly conversion to TAI for converting
to and from JD.  This is more expensive but makes these formats work
like any other format and not be restricted to a particular scale.