Added support for scalar ITensor. Streamlined some code in itdata.h, such as removing Curry function objects in favor of lambdas. Added detail::call function template which converts the error of an object not having an operator() method into a runtime error instead of a compile-time error.
Finished out some missing features needed to support scalar ITensors, such as contraction with scalar, visit, addition, multiplication of scalar ITensor by scalar, etc.