src/filament/sever_fila.jl
MEDYAN.sever_fila!
Return the tag of the new filament plus end.
The split will happen between place and the monomer slightly towards the minus end.
The split cannot create a filament with less than 2 monomers.
Each daughter filament keeps the load force of its surviving original tip; the two newly created tips at the cut site get load force 0.