commented /utilities/option.cc

First comments for ITensor in /utilities/option.cc.  Geared towards the broadeest possible audience.
In this comment and the similar ones that follow, let's not refer to the Val type, since it's an internal detail of how the Args class works. Instead, we should say that getBool gets the value of the named argument "name" assuming it is of boolean type. Also need to mention that this version throws an error if there is no argument found with name "name".