Implement neat lead interfaces for irregular systems

Current State of Affairs

Currently in Kwant when one attaches leads there have to be sites added to the system so that the interface is "flat". Although the additional sites do not affect the physics, it is an eyesore. This is particularly noticeable in graphene nanoribbon systems, for example.

Proposed Solution

The essential reason for the above problem is that the fundamental domain of the lead is chosen before the lead is attached to the central system. In principle a better choice of fundamental domain could be made when attaching the lead, so that the interface "fits" with the shape of the system.