Use float for colorbar recalculation in PyplotViewer
PyplotViewers all exit gracefully on sigint

Now it is not needed to surround all the code which works with PyplotViewer
instances into a try-except and catch KeyboardInterrupt. This is done
automatically by handling the appropriate signal in concert.ext.viewers module.
Terminate all pyplot viewers on normal exit
PyplotViewer refactoring
Merge pull request #104 from ufo-kit/pyplotviewer-upgrage

Pyplotviewer upgrage