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.

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

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

Modify tutorial to remove warnings.

Closes #304.

Closes #279.

Closes #214.

Closes issue #299

Currently we just set the default figure size. Matplotlib is
annoying when it comes to setting these default parameters, and we
have to ensure that we set any 'rcParams' in a separate Jupyter cell
and *after* we import pyplot.

Use a more modern sphinx invocation