The Group now has generic Node building methods.

The Group used to have hard coded methods to build certain
node types.
This is no longer the case. The generic build method will
take a builder id which specifies which builder to use to
build up the node.

The init() methods from nodes has been removed.
This used to typically be used to setup the plugs on the node.
This chore is now being done in the builder for the node.
This consolidates all the building tasks to the builder now.

Still need to do a bit more cleanup on the ussage of get_spoke<>.
We should also add a python example of how to create a builder.
But we still need to expose the builder framework objects to python.