This commit contains many changes made by Haocheng, David and myself.
It performs the following:

a) move operators out of model and have them operate indpendently from
   model sessions
b) operators now each have their own attribute collection, rather than being
   represented as single attributes from an attribute collection
c) Separate XML descriptions from the base operator, loosening the requirement
   on operators to simply have a descriptive attribute (XML descriptions are
   now supported as a subclass of the base operator). This facilitates the
   construction of python operators without requiring an XML string describing
   their inputs
c) introduce an operation manager that tracks the availability of operators.
   It has the following properties:
   i.   Registration of operators via a unique name (usually the c++ name of
        the operator)
   ii.  Provide a connection to a resource manager so that operations that
        construct resources will automatically register th...