AtB: Assign Heir

[VampireSeraphin]

In AtB, when a pilot is retired or killed they take their mech with them. The XLS rules say that if they have an heir, the unit passes to that NPC instead. However, right now, that status cannot be assigned.

An option should be added that will allow the player to designate an heir. The unit would then pass to that character using a variant of the original unit rules already in place. If the heir already is assigned to a unit, the new unit should add an entry to the unit history log instead & remain in the hanger.