Small merge request towards simplifying kpoint handling

In different places, the response code passes kpoints as index or as coordinates. We'd like to avoid passing them as coordinates but instead have them as traceable objects (essentially indices). This MR "bubbles up" kpoint processing so the caller is responsible for deciding and passing a kpoint index instead of passing raw coordinates around.

(We can think of this as a special case of "normalize as early as possible".)

The MR is not a "complete" change because we cannot yet remove the logic we are trying to get rid of, but it's one step which I think is mergeable. Changes:

• bubble up kpoint decisions from low-level (near "factory") to high level (chi0 objects etc.)
• pdens tool: Use Domain object to clean up logic
• move pdens test to gpwfiles