Species in which molecules are each made of a single spherical atom.
Does not permit multiatomic molecules. The advantage of this species
over the multiatomic version (used with 1 atom), is that one layer of
the atom hierarchy is eliminated in SpeciesSpheresMono. Each atom is
the direct child of the species agent (i.e., each atom is at the "molecule"
level in the hierarchy, without an intervening AtomGroup).