MAINT: removed unused flatten_tree
MAINT: removed unused flatten_tree

mnicholl/MOSFiT,villrv/MOSFiT,guillochon/MOSFiT,mnicholl/MOSFiT,guillochon/MOSFiT,villrv/MOSFiT,guillochon/FriendlyFit,bmockler/MOSFiT,bmockler/MOSFiT,mnicholl/MOSFiT,guillochon/MOSFiT