Add ptasks in the Python bindings

Add ptasks in the Python bindings

- This adds several functions in the Python bindings used for manipulating
  ptasks, such as parallel_execute or exec_init.
- This also adds the Engine.set_config function, needed for changing the
  configuration to 'host/model:ptask_L07' in the script.
- Finally, an exec-ptask.py example is added, exact translation of the
  C++ example s4u-exec-ptask.cpp.