onnxruntime

Document the Graph header files and cleanup some issues. (#42)

* Checkpoint.

* Add doco to graph.h and graph_base.h.
Change NodeConstIterator to return a reference to clearly advertise no nullptr's are going to be returned as it's only iterating valid Nodes.
Fix some code analysis warnings.

* Make a couple of APIs return a reference instead of a pointer as they never return nullptr.

* More doco and some minor naming cleanups.

* Cleanups
Couple more consistency changes.

* Fix CUDA test file

* Fix invalid line.