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.

Fixup/qsymm tests

See merge request kwant/kwant!330

sympy symbols with 'real' set differently do not hash to the same
value (and possibly do not compare equal).

__eq__ for Models just uses allclose anyway.

This is more idiomatic.

Also comment other usages of this parameter.

Qsymm is now installed in the testing image directly

Not providing these is now deprecated.
Close #325.

fix kpm.conductivity in the case where 'norbs' is not set

See merge request kwant/kwant!321

This does not require assuming that site families have 'norbs' set.

Closes #313.

fix a bug in kwant.qsymm

See merge request kwant/kwant!325

The test suite uses features from the 1.2 release of qsymm.

This is not an API of qsymm.

Closes #305.

This reverts commit f2ac01b9.

We still have to manually start image building jobs, and there is
no disadvantage to allowing rebuilding arbitrary images.

Latest versions of Kaniko builder use this new credentials directory

Sphinx 2 got rid of 'sphinx.main' so we can no longer use it.