Improve treatment of value functions in the tutorial
In the Kwant tutorial value functions are introduced in the section "Spatially dependent values through functions" of "More interesting systems: spin, potential, shape". The first presentation is without parameters and the (equally important) use as system-wide parameters is quite hidden. I suggest to rename the section to "Using function as builder values" and explain both usages on equal footing.
Also, we should switch the complete tutorial from args to params.