make plot more useful for low level systems

doc/source/images/tutorial1a.py

doc/source/images/tutorial2a.py

doc/source/images/tutorial2b.py

doc/source/images/tutorial2c.py

doc/source/images/tutorial3a.py

doc/source/images/tutorial3b.py

doc/source/images/tutorial4.py

doc/source/tutorial/tutorial1.rst

doc/source/tutorial/tutorial4.rst

doc/source/whatsnew/0.2.rst

examples/tutorial1a.py

examples/tutorial1b.py

examples/tutorial2a.py

examples/tutorial2b.py

examples/tutorial2c.py

examples/tutorial3a.py

examples/tutorial3b.py

examples/tutorial4.py

kwant/plotter.py