Allowing S- and S+ in addition to Sp and Sm in spin Models. Added S2 (S squared) operator to both.
Updated ExtendedHubbard.h to use new Model interface and changed design of kinetic energy part.
Updated sample/exthubbard to use latest InitState/Model interface.
Added back named operator methods to Hubbard Model class for backwards compatibility
Added feature allowing operator strings of the form Op1*Op2, telling Model objects to return the product of Op1 and Op2.

Fixed conflicts:
	itensor/model.h
Updated exthubbard sample code and input file to use latest library interface.
Added back older Model class interface to SpinHalf, SpinOne, Spinless, and tJ models for backwards compatibility.
Moved recently added endif to just below initial ifdef I