Merge branch 'refactor/plotter' into 'master'

Refactor 'plotter.density' into 'plotter.scalarplot' and
fix up plotter docstrings.

Now 'plotter.density' is just the composition of
'interpolate_density' and 'scalarplot', which makes it easier
for users to customise if they need.

See merge request kwant/kwant!223