One less import, and the explicit pattern match is not any less readable

The removed logic is an artifact from when vectorized and
unvectorized systems shared the same logic for 'hamiltonian_submatrix'.
This was accidentally left in when we reverted to the old logic
for unvectorized systems.

We forgot to add him when merging kwant/kwant!333

Require that norbs are specified for abstract vectorized systems.
This was already a de-facto requirement for finalized Builders, and
it should be part of the abstract base class.

Closes #336

See merge request kwant/kwant!334

All site arrays must have 'norbs' set by definition.

The docstring erroneously stated that 'site_ranges' could be 'None',
however we want to restrict VectorizedSystems to contain only site
families that have 'norbs' specified. The docstring is now explicit
about that fact.

fix add_peierls_phase for system without leads

Closes #331

See merge request !333

Incraese the timeout and number of retries when running the
link check on the documentation; this should increase the
specificity of the test.

See merge request kwant/kwant!332
Closes #332.

The combination of longer timeout and larger number of retries should
make the linkcheck sufficiently specific again.

Retrying > once should increase the specificity of the check a lot.

vectorize systems so that value functions may be called once
on a whole vector of sites (for onsite value functions) or a pair of
vectors of sites (for hopping value functions). In this way we avoid
the Python overhead of calling the value functions many times. For systems
with vectorization enabled this can result in a ~100x speedup when evaluating
the Hamiltonian.

See merge request kwant/kwant!323
Closes kwant/kwant#302

Update internal uses of 'hamiltonian' to suppress this warning
and add a TODO to update to the new API.

We now have *System and *System2, so the old 'isinstance' checks
will not be complete. We add convenience functions in 'kwant.system'
and 'kwant.builder' to check against all the relvant classes.
We leave the original 'isinstance' checks in 'kwant.physics.gauge'
because that module does not work with vectorize systems yet.

These will be used in vectorized systems later.

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.

Now 'which' and 'act' (and, by extension, 'to_fd') now accept
SiteArrays as well as Sites.

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

Previously the value function would return a single-element vector
(which is illegal); now it returns a scalar.

bump the required versions of dependencies

See merge request kwant/kwant!331

This fix will need to be backported to stable too, but we want the
testing images to be built on master, as other requirements
(specifically doc building) have changed.

It has been > 6 months since the release of Ubuntu 18.04 LTS.