to fix #39 also in Python2

As PyState_AddModule is not yet called in the module definition,
PyState_FindModule will not work, so for Python 3 we need to get the
module from the methods definition to retrieve a pointer to dgpyError.

Fix #39

This allows accessing the list of entities instanciated in the dynamic-graph.

This allows accessing the list of entities instanciated in the dynamic-graph.

  - add isPlugged and getPlugged methods,
  - add property getter name.

    Use PyObject_IsTrue to test a boolean in C/Python API,
    call print as a function.