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

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

ref https://github.com/stack-of-tasks/dynamic-graph-python/pull/34#discussion_r340106255

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

pointed out by OS.

This is crucial when debugging remotely using winpdb.

This corrects the issue detailed here:
https://github.com/jrl-umi3218/dynamic-graph-python/issues/2

Solution found by Florent Lamiraux.