Communicator accessibility

[apeyser]

Currently communicators are internal structures. It would be helpful to pull out the communicator, particularly for the case of MUSIC which uses a sub-communicator as part of putting as much of the machinery outside of NEST rather than buried in internal structures.

So either by integrating pymusic:
s = music.Setup()
s.getcomm()

or by adding a nest call:
nest.getcomm()

[jougs]

NEST Core came to the conclusion that this is too unspecific and should be closed.