closes #125

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

This was deprecated several releases ago

Current implementation uses ``kwant.physics.Bands``. This example fails
when one substitute ``k_x**2`` with for example ``k_x**4`` due to farther
then nearest neighbours hoppings. Using wraparound provides also easier way
to extend it to multidimensional band structures (except of plotting).

The Mathjax CDN is shutting down:
https://www.mathjax.org/cdn-shutting-down/

Closes #116.

Closes #113.

the '_defs' module is used solely when generating images
within the tutorial part of the documentation.

Uses the new conservation law and discrete symmetry interface
to Builder, and illustrates the separation of the scattering matrix into
blocks depending on conservation law eigenvalue.

Also improve the documentation for operators in general.

special methods are not included by autodocs by default, and
as class documentation pages are generated by autosummary,
we cannot just explicitly include __call__. We get around this
by using a custom autosummary template for the operator classes.

For cython extension modules, the method signature for __call__
is still not embedded properly.
See https://github.com/cython/cython/issues/1577.