add 'parameters' property to all system/lead implementations

Specifically: finalized Builder, PrecalculatedLead,
              ModesLead and SelfEnergyLead

This can be used to inspect the parameters on which a system depends,
and can be later used to give more informative error messages.

Adding 'parameters' to ModesLead and SelfEnergyLead is strictly a
backwards-incompatible change, however this is an advanced feature of
Kwant and we don't want to let this block adding 'parameters' to the
system API.