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 the new resources to
        the manager
   iii. Provide a list of available operators given an input selection (i.e.
        show which operators accept a given input)
d) allow read/write locks on operations' input resources and add tests for
   asynchronous operations
e) break static registration of operations and resources into a set of
   libraries that accompany the libraries in which they are defined. A library
   that provides static registration of a resource and associated operations
   for library smtkFoo is called smtkFooEnvironment, and the registration
   occurs in the foo::environment namespace
f) deprecate the exodus session
g) convert smtk::model and polygon, mesh and discrete sessions to use
   nlohmann::json instead of cJSON. With this change, we can hopefully remove
   cJSON from smtk
h) provide operations for creating, loading and saving smtk resources
i) update tests to use the new operation framework

This merge is a work in progress, but has become too large to avoid merging
into the msater branch. The outstanding issues are

a) remove smtk::model::Operation from smtk (it is still required for the qt
   bindings)
b) rename smtk::operation::NewOp smtk::operation::Operator
c) fix Qt bindings to use the new operation infrastructure
d) fix the infrastructure for meshing interop with remus to avoid using json
   fragments
e) remove cJSON from smtk
f) switch resources' read/write/create methods from bare functors to smtk
   operators. This is more in keeping with the pattern of using operators to
   affect resources
g) enable CGM session
h) enable matplotlib extension