Inserts an Atom into the array at the specified position.
If the index equals the number of items in the array then the
Atom is appended to the end.
Atoms can exist in only one place at a time. If the Atom
supplied is already contained by another object then a Clone of the Atom is
Adding a null value will result in a NullAtom being added to the
If the index is not a valid index this method throws an