phenopacket builder should have function to update phenopacket

[pnrobinson]

Something like

PhenopacketBuilder::update(Phenopacket previousPhenopacket) -> Phenopacket

that is, we would use the PhenopacketBuilder to gather dataq about the current visit and then we take a Phenopacket object and add the new information to the PhenopacketBuilder, and then output the builder.

This includes items such as

1. Add update to metadata
2. Do not add duplicates, but consider adding evidence (e.g.., that a certain HPO term was reported on two visits to have the same onset)

[pnrobinson]

redundant with #54