Length of the linker in Angstroms. The length of the linker is defined
as the length of the vector from the point of attachment on the biomolecule
to the center atom of the fluorophore. The center atom of the fluorophore
is identified in the _flr_chromophore.center_atom data item.
The point of attachment in the biomolecule is specified by one of the
following data items (whichever is relevant).
_flr_poly_probe_position.atom_id
_flr_poly_probe_position_mutated.atom_id
_flr_poly_probe_position_modified.atom_id