commented /utilities/option.cc

First comments for ITensor in /utilities/option.cc.  Geared towards the broadeest possible audience.
Here I would say "Add a named argument having either a boolean, integer, string, or Real value to this Args object". You can leave out the bit about (constant). The type `const char*` is the type of a "string literal" i.e. a string written directly into a program; it's still a type of string, just not a std::string (i.e. a C++ standard library string).