gedcom datestrings to dates, including for ISO 8601 dates

[andersbel]

Some gedcom files have dates formatted in ISO 8601 format. Some genealogy software, for example Gramps, parses this correctly.
It would be nice if python-gedcom can parse datestrings to date objects, including datestrings in ISO 8601 format. Then several methods for working with dates become available without having to be implemented again.
The necessary functionality is available in Python 3.7 but forcing an upgrade to this version seems premature. Using the package python-dateutil seems to be a viable option. It can do all of this and is available for Python 2.7 and up.

[andersbel]

One function that is useful and will be available when using python-dateutil is to compute difference between two dates as number of years, months and days. Information about the age of a person in these units, at the time of death, is sometimes available in Swedish church records. Thus it is useful to have a function to compute it as such, and the possibility to compare with information from records. It is not always correctly computed in the church records though.