src/sever_filament.jl
MEDYAN.sever_filament!
Return the new filament id of the minus end filament.
Calling this will invalidate the chem cache.
The split will happen between mon_name and the monomer slightly towards the minus end.
The plus end will keep the same fil_id, the minus end will get a new fil_id.
The split cannot create a filament with less than 2 monomers.