Parents

Add parents to a member who does not have any parents.

Regarding to generating multiple new member keys, I decide to go with the take(n) approach rather than

• pass in a pedigree object, which means we go through existing keys twice in the add parent function.
• or maintain a list of used keys, which add more objects we need to keep in sync with redo/undo.

However, take(1) returns a list, which breaks existing code. We can make _newMemberKey() return a string if n == 1 or a list if n > 1, but I don't like to have variable return type. Instead I created a simple wrapper function for the n == 1 case.