After #77 (closed) the call signature of kwant.smatrix and other consumers of System have a rather lengthy call signature: kwant.smatrix(syst, energy, args=..., kwargs=...). It is counter-intuitive to supply arguments that the system consumes using awkward syntax and via an unrelated function.

I propose an alternative pathway for doing so. A method System.update_parameters(*args, **kwargs) that will return a shallow copy of the system that stores its arguments and keyword arguments such that the user doesn't need to provide them elsewhere. Example usage:

[kwant.smatrix(syst.update_parameters(0, B=5)) for B in fields]

Extra considerations

• Systems already have a method precalculate that returns a modified shallow copy.
• This change is fully backwards-compatible (old way of providing parameters would still work).