Factor out `kwant.kpm`
Anton suggested that it would be better in the long term to separate kwant.kpm
in its own package, so other projects that rely on a general KPM implementation don't need to import all of kwant. This would also make it more transparent to add improvements to the KPM implementation that are independent of kwant (for example I'm working on implementing KPM in a non-orthogonal basis now, something that kwant doesn't support yet).
What do you think @anton-akhmerov @cwg @pablopiskunow ?