The number of linker atoms in the probe.
The number of linker atoms is counted starting from the atom specified
by one of the following data items (whichever is relevant in the
particular scenario) and ending with the point of attachment in the
fluorophore.
_flr_poly_probe_position.atom_id
_flr_poly_probe_position_mutated.atom_id
_flr_poly_probe_position_modified.atom_id
The linker also includes the reactive group of the fluorophore and any
modifications carried out to introduce a spacer and/or an internal
linker within the probe.