Some cleanup of IQIndex. Moved method offset outside class and removed unused biggestm.
Made IQIndex a subclass of Index. Makes code shorter and less error-prone. IQIndex "really is" an Index but with additional structure.
Fixed error check on density matrix to be more correct, less strict. (Nothing strictly wrong with having multiple blocks with the same quantum numbers.)
Updated Counter inline docs to reflect 0 indexed behavior.
New version of ITensor::expandIndex which uses a simple loop with an offset.