Modify the prescription for time_origin format

It turns out that the DALI prescription for ISO-8601-like timestamps is not quite suitable here since it can't cope with dates BCE, so the time_origin value for time_coords_id='jd' in Appendix B would not be compatible.

So remove reference to DALI and just present a pattern that should be comprehensible to readers, with a vague reference to ISO8601.

I have also removed the (optional) 'Z' timezone indicator; DALI says "Astronomical values never include a time zone indicator", allowing Z only for "civil" dates (see DALI v1.1 sec 3.3.3).

Finally I removed the optional ".000" from the predefined values, since they don't provide much value. Users can still supply fractional seconds if they want to, as should be clear from the examples.