Add a date attribute for PartialUncertainOrApproximate

[rettinghaus]

If you parse a date with qualification like '2004-06~-11' or '?2004-06-~11' it's quite complicated to get the date without qualifiers. UncertainOrApproximate dates like '2004-06-11%' have a date attribute to get '2004-06-11', which is missing for PartialUncertainOrApproximate dates.

[rettinghaus]

I understand now the reason for this and why upper_strict() and lower_strict() is better suited.

Yet the problem why I stumbled upon this is:

isinstance(edtf.parse_edtf("~1812-04"), edtf.parser.parser_classes.Date) evaluates to true, but asking specifically for uncertainty fails:

>>> edtf.parse_edtf("~1812-04").is_approximate
False
>>> edtf.parse_edtf("~1812-04").is_uncertain
False
>>> edtf.parse_edtf("~1812-04").is_uncertain_and_approximate
False

And also there is of course no date attribute.

[rettinghaus]

relates to #31