Defines the placement of the molecules. Atoms in all lists are
considered for placement. Subclasses define the specific algorithm
for placement, and for interpretation of the different lists.
Some subclasses will simply place all atoms as if they are in a single
list, while others will place atoms in different lists in different
ways, for example to form a lattice appropriate to a compound
(i.e., a solid-box mixture).
atomList - array of list of molecules to be placed by this class