The chiral volume, V(c), for chiral centres that involve a chiral
atom bonded to three non-hydrogen atoms and one hydrogen atom.
Vc = V1 * (V2 X V3)
V1 = the vector distance from the atom specified by
_chem_link_chir.atom_id to the first atom in the
CHEM_LINK_CHIR_ATOM list
V2 = the vector distance from the atom specified by
_chem_link_chir.atom_id to the second atom in the
CHEM_LINK_CHIR_ATOM list
V3 = the vector distance from the atom specified by
_chem_link_chir.atom_id to the third atom in the
CHEM_LINK_CHIR_ATOM list
* = the vector dot product
X = the vector cross product