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.

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

Minor changes in matlab display. Add a functor member to explicit the callable of optionalparentethis.

    global variables g_pool and g_factory have been replaced by
    PoolStorage::getInstance and FactoryStorage::getInstance static methods.