Later these will become part of the official System interface

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.

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.

closes #283

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

This has a lower chance to give an impression that the substitution
happens in-place.

Closes gitlab issue #237