Add support for ranges with open start/end date

[JeroenDeDauw]

While we support ranges with open start or end like ../~2004-06-20, we do not support start or end that has a date and is open at the same time, like ..2004-06-01/~2004-06-20. While the later is not part of the public EDTF spec, it appears as example in a proprietary spec.

[JeroenDeDauw]

From @mzeinstra:

Level 2: Date: Combining advanced Qualification with range element expansion and time intervals.

Example: '..2004-06-01/~2004-06-20 (beginning before or on 1 June 2004 and ending approximately the 20th)

[JeroenDeDauw]

@mzeinstra are we sure that is a valid EDTF value? I'm not seeing any combination of .. (open interval) and dates in https://www.loc.gov/standards/datetime/. It also looks like other implementations do not support this.

[mzeinstra]

I've put it on the mail to Claude and Lucien to answer this questions.

I've not seen the formal EDTF standard that they have been using: https://www.iso.org/standard/70908.html

[JeroenDeDauw]

Now confirmed: this is valid EDTF, even though it is not listed in https://www.loc.gov/standards/datetime/

Our current object model cannot express those dates, so adding support is non-trivial (though also not that much work).

[JeroenDeDauw]

More examples in e328f53

[mzeinstra]

I am not quite sure I understand why these dates fail in the date representation.

I see that dates with .. seem to proces in the Valid Strings Ln92,96,100 do validate right?

		yield '[..1760-12-03]';
		yield '[..1984]';
		yield '{..1984}';

so why would the following not work?

//		yield '..1984';
//		yield '1984..';
//		yield '1670..1673';

[JeroenDeDauw]

Replied with #46