Allow x and y dist mappings simultaneously

[mjskay]

e.g. make the internal dist-based mapping get a directional component (xdist and ydist); dist is automatically rewritten to this depending on orientation. Would make it easier to make #43.

Currently we just re-map the directional version to dist on the way in inside setup_data() / compute_panel(); need to change this to map dist to the appropriate directional version instead and then actually use the directional version elsewhere internally.