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

As, from the docs:
> Passing arguments of the wrong type (e.g. passing a list when an int
> is expected) should result in a TypeError.

Thanks @jmirabel

thanks @jviereck

While here, develop test from previous commit, and fix another test in
the same file.

fix #36

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.