Change to Structure/Molecule API. Now both inherits MutableSequence and
implements __setitem__ and __delitem__. Effectively, Structure and Molecule now
functions like Lists. You can now do the following:

structure[0] = "Fe"
structure[0] = ("Fe", [0.5, 0.5, 0.5])
structure[0] = PeriodicSite(....)

Same API for molecules.

Even better, now list functions like reverse, extend, sort are automatically
part of Structure and Molecule.