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

Additionally update all associated docstrings to say that 'args'
is deprecated in favor of 'params'.

This means we don't need to create a dummy class that implements
__get__ just so we can use hamiltonian_submatrix as a method.

Closes #270.

Also require that 'leads' of FiniteSystems have a 'parameters'
property.

Specifically: finalized Builder, PrecalculatedLead,
              ModesLead and SelfEnergyLead

This can be used to inspect the parameters on which a system depends,
and can be later used to give more informative error messages.

Adding 'parameters' to ModesLead and SelfEnergyLead is strictly a
backwards-incompatible change, however this is an advanced feature of
Kwant and we don't want to let this block adding 'parameters' to the
system API.

There is nothing that consumes lead_paddings right now, but it will be useful
for plotting and gauge fixing.
Closes #266.

Previously it was never explicitly stated that the wavefunctions
returned by modes() are only defined over the unit cell of the
infinite system. This was confusing, as syst.sites contains
not only the unit cell sites, but also the connected sites in
neighboring cells.

Closes #178.

Hamiltonian value functions may now have signatures that depend on
different parameters. On finalization, Builders now inspect all of their
value functions and store the names of the parameters on which the value
function depends. Then, when evaluating the Hamiltonian, a particular
value function is only passed the parameters on which it depends.

'System.hamiltonian' has been updated to accept an extra keyword-only
parameter, 'params', which is a dictionary mapping parameter names to
values. This is mutually exclusive with the existing '*args'. All
top-level API that takes an 'args' parameter now also takes a
keyword-only parameter, 'params'

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

this fixes #35

Unbound methods are not longer a thing in Python 3 [1].  Instead,
functions use the descriptor protocol [2] to properly bind the `self`
argument if they are being called as a method. Cython functions do not
have a `__get__` attribute and so don't satisfy the descriptor
protocol. They therefore can't be used as methods. As a workaround we
create a wrapper class with the proper __get__ method. The advantage of
this over creating a Python wrapper is that the function signature is
preserved.

[1]: https://docs.python.org/3.0/whatsnew/3.0.html
[2]: https://docs.python.org/3/howto/descriptor.html

Duck typing generally works well, but it can lead to very confusing error messages.