This has a lower chance to give an impression that the substitution
happens in-place.

Closes gitlab issue #237

Previously we used the matplotlib default behavior of setting
the colormap limits (vmin and vmax) to the limits of the plotted
data, if they are not provided by the user. Now we set the
colormap limits to the 2nd and 98th percentile of the input
data, and stretch the limits by 10% on either end in an attempt
to include all the data in the limits. If we cannot do so
we set the limits to the percentiles plus the stretch
and issue a warning.

Also, whenever the data falls outside the colorbar limits
(even when the latter are set by the user) we set the appropriate
ends of the colorbar to be pointy, indicating that the data
extends beyond the colorbar.

Closes #183

Thanks to Tibor Sekera for suggesting this feature.

Closes #166.

I assume that most people encounter the tutorial example scripts by
reading the documentation, and not by viewing the tutorial subdirectory
of a checked-out Kwant source.

That's the motivation for moving all the manipulations of tutorial
scripts from setup.py to doc/Makefile.  Previously, a successful 'make
html' would require a preceding execution of 'setup.py build_tut'.  Now,
a simple 'make html' is enough.

While at it, I reorganized where the example scripts and their outputs
are stored.  Everything is now in 'doc/source/code' with its three
subdirectories 'download' (to be shown to readers), 'include' (with
include markers), and 'figure' (figure generation & figures).  This
organization is clearer and also makes the generation of figures
separate from the tutorial.

The site() method was never widely advertised, so no one will miss it.

This was deprecated several releases ago