Add the 'subgraphs', 'terms', and 'site_arrays' attributes, and
'hamiltonian_term' method and implement 'hamiltonian_submatrix',
'cell_hamiltonian' and 'inter_cell_hopping' in a vectorized way.

We remove the ABC metaclass as now 'normalize_tag' and 'normalize_tags'
are cyclically defined, and subclasses must redefine at least 1.

Later these will become part of the official System interface

Also fixup some docstrings for better formatting.

Previously 'plotter.density' was not as well factored as
'plotter.current', which made it hard to compose, e.g. for
passing custom parameters to 'plotter.interpolate_density'.

Also add reference documentation for 'density'.

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

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

These classes used not to have any features beyond
kwant.system.{Finite,Infinite}System, but this is no longer the case.

In addition to basic updates to skip permanent redirect, remove an
obsolete reference to macports installation instructions on the main
website.

Closes #113.

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.

Summary of changes:
* Add a test for trailing whitespace, remove all trailling whitespace
* Add a test for broken URLs in docs, fix all broken URLs and permanent redirects
* Switch sphinx doc build to strict (warnings to errors) and nitpicky (broken links to warnings). Fix all sphinx warnings.
* Deploy docs from any commit to [kwant/kwant](https://gitlab.kwant-project.org/kwant/kwant) to https://test.kwant-project.org/doc/dev
* Deploy docs from any commit to master at kwant/kwant to https://kwant-project.org/doc/dev
* Deploy docs from any tag release tag (so anything that is vX.Y.Z) at kwant/kwant to https://kwant-project.org/doc/X.Y

fixes #36

This undoes 00b0536c, reaching the aim of clearer error messages in a more
systematic way.