Made AutoMPO conversion to MPO and IQMPO no longer explicit to allow writing code such as IQMPO H = ampo; Updated sample codes to all use AutoMPO where possible, and use new Args interface.
Moved definition of NAN macro to types.h
Made InputGroup constructors more flexible. Now InputGroup uses optional_ptr internally to manage either its own InputFile or an externally owned one. Changed behavior of get___ methods with no default to throw an exception if that input parameter is not provided.
Added a new constructor for VectorRefNoLink for use in a new experimental tensor contraction code.
Updated utilities/Makefile to include optional_ptr
Fixed Sweeps class to use updated Args interface.
Renamed OptSet and Opts to Args in sweeps.h
Renamed OptSet and Opts to Args in dmrg.h