Possible improvements to GestationalAge

1. [Age](https://phenopacket-schema.readthedocs.io/en/latest/age.html) is represented using an ISO 8601 duration, but [GestationalAge](https://phenopacket-schema.readthedocs.io/en/latest/gestational-age.html) has key/value pairs for weeks & days.

It would be more consistent to represent them both the same-- that is, GestationalAge of 33 weeks and 5 days could have iso8601duration of 'P33W5D', avoiding clients from having to deal with these ages as different formats.

2. It may also be important in some contexts to record the method by which gestational age was assessed (e.g., by LMP, ultrasound assessment, date of IVF embryo implantation).