* Modify TBB graph nodes creation and deletion
* Add a graph* member to CPURuntimeContext.
* Create nodes the first time a function is called, all the following calls only exectue the computation.
* Delete nodes when cleanup_runtime_context is called.

* Add TBB global_control* and task_scheduler_init* members to CPURuntimeContext.

* Remove one comment.
Do not write two TBB header files and one #define to generated c++ source code.

* Move TBB header file and #define before other header files in generated c++ source code.

* Move one comment to the top in generated c++ source code.