Rework the date/time parsing to tighten up some cases and to enable other

Rework the date/time parsing to tighten up some cases and to enable other
 cases which should have worked but did not.
Now supports julian day (J2452271), ISO time labels (T040506) and various
 combinations of spaces and run-togethers of dates, times, and time zones.
All regression tests pass, and I have more tests to add after the 7.2
 release (don't want to require changes to the ancillary horology result
 files until after then).