Update "IndexSet" docs for V3

[mtfishman]

Some of the IndexSet docs (http://www.itensor.org/docs.cgi?vers=cppv3&page=classes/indexset) need to be updated more for V3:

• .r and rank -> order
• Update findIndex functions to only accept a TagSet for matching once prime levels can be accessed through the tagset.
• maxM and minM -> maxDim and minDim
• Move "Other IndexSet Functions" to the top since I think those are ones people would use most. The other functions are mostly internal ones that get used when they are called on an ITensor.
• Make the "Accessor Methods" free functions?
• Rename extent -> dim?