Add density plotter, the analogue to 'kwant.plotter.current'
for scalar fields.

Closes #157

See merge request kwant/kwant!214

Also add reference documentation for 'density'.

This helps distinguish where the system is and results in plots
that are easier to interpret.

Now it will be relatively straightforward to factor out
_interpolate_field to Cython or C.

This is copy/paste from 'interpolate_current' with minimal changes.

Allow Builders to substitute parameter names

Closes #142

See merge request kwant/kwant!215

This change is backwards compatible and will make the intention
more clear when addressing the output.

Thanks to Alexandre Bernard for reporting this.
https://mailman-mail5.webfaction.com/pipermail/kwant-devel/2018-May/000088.html

Because of the incorrect name, the example in the docstring of lamdify
was not even visible in the documentation.

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

Closes #171

Decrease padding to half the bump width and add test that the
current goes to zero at the boundaries of the region

See merge request kwant/kwant!211

This checks that the padding around the counding box is sufficient.

The bump function we convolve with has finite support,
so the vector field is identically zero at distances
greater than width/2 outside of the bounding box.
Also replace this magic number with a 'padding' constant
to improve readability just a smidge.

This can always be resurrected from Git if it is really needed,
but at the moment its just 3000 lines that aren't used.

speed up kwant import from >1s to 0.3s

Closes #181

See merge request kwant/kwant!207

Previously we used the ExtensionUnavailable proxy that would
raise an exception when accessed. Now because kwant.continuum
is loaded lazily we can just raise an exception. We choose to
re-raise ImportError with a better error message.

'matplotlib', 'Figure', '_colormaps', 'mpl_available', 'mplot3d',
'has3d', 'LineCollection', 'PathCollection', 'Line3DCollection',
and 'Path3DCollection'.

We will need to issue deprecation warnings when using this with
'kwant.continuum', so we have to roll our own so we can hook into
the attribute access. When we no longer need this feature we
can replace this with a simpler implementation using
'importlib.util.LazyLoader'.