Extension of ASSO tag

[webtrees-pesz]

I think it make sense to add a DATE substructure to the ASSO tag.

Examples:
0 @i1@ INDI
1 ASSO @void@
2 PHRASE Albert Einstein
2 ROLE OTHER
3 PHRASE Doctoral advisor
2 DATE FROM 1930 TO 1932

or
0 @i1@ INDI
1 ASSO @void@
2 PHRASE Franz Freese
2 ROLE OTHER
3 PHRASE Neighbour
2 DATE 1980 TO 1986

[tychonievich]

Discussed in steering committee.

• We agree that this is a reasonable addition.
• We think that the flexibility of DateValue is useful over just DatePeriod, to allow things like ABT 1880 and associations that occur in a single moment.
• There's a small typo: the second example is missing the FROM part of the date period.
• It's not clear how widely this is used now, likely in NOTEs because of the lack of DATE. Because the elements of this are already present in the spec and the information proposed is stored in some existing data in another location, we don't think we need to add the "awaiting use" label and plan to add this to the v7.1 branch. This follows the precedence set by adding RESN to more records without the "awaiting use" criterion.