Function zoomPanProfile

Van Wijk & Nuij's optimal zoom-pan profile between a view of height w0 and one of height w1 whose centres are travel apart. at(f) returns, for the fraction f of the path's length, how far along the travel the view centre is (along, 0 → 1) and the view height there. S is the path length in their units; pure zooms and no-ops are handled. Mirrors d3-interpolate's interpolateZoom.