No limit of hitpoints per cure/heal, heal at start of healer turn.
Healing still doesn't stack (you can only be healed by one thing), but
standing next to allies healer can get you double-healing.
If healers are now too powerful, simplest would be to limit number of
patients each healer can treat.
If allied healers are too powerful, we could extend limit across
entire turn, so unit cannot be treated by more than one healer per
turn, even allies.
