Since this does not seem to justify a new bugfix release of Kwant 1.3,
I apply this commit to the 1.4 stable branch.

When System.parameters was introduced, we thought it would be a good idea to
also use this new attribute.  The plan was to check in hamiltonian_submatrix()
whether any unneeded parameters are provided by the user.  This turned out to
be impossible, so the only check that remained was whether all the necessary
parameters are indeed provided.  However, this check is not necessary, because
the 'hamiltonian' method is in any way obliged to signal an error when it does
not obtain the necessary parameters.

This allows static analyzers like "Jedi" to work.
Closes #223

closes #283

This is a workaround for
kwant/kwant#280
and should be reviewed once that issue has been dealt with.

In order not to complicate the licensing situation of Kwant proper,
the new logo will be maintained in a separate repository.

This is a partial application of the changeset proposed in
kwant/kwant!274,
which removes the need for a 'conftest.py', instead using
'pytest.importorskip'.

We apply this change only to 'qsymm', as applying this change elsewhere
requires rethinking where tests are put in the package structure of
Kwant. This minimal change allows us to build packages for Conda and
Debian, which is necessary for releasing Kwant 1.4.

This allows subclassing of operators by third party libraries
(e.g. tkwant).

This shouldn't be a problem, but sphinx complains.

'temp' is too short a parameter name, especially for an API,
and could be confused with 'temporary'.