Refactor matplotlib backend.

- Install matplotlib backend to gr.matplotlib.backend_gr
- Move examples to examples/matplotlib
- Added GROPTS environment to start scripts

Change-Id: Ida81eb3fc7b5dd459bd1b4c46ed63e0420a94370
Reviewed-on: https://iffwww.iff.kfa-juelich.de/review/63
Reviewed-by: Josef Heinen <j.heinen@fz-juelich.de>