fix-333-allow-dimension-based-distances

[timothypratley]

Convert dx*dx into a property function so that it can be overriden but users who would like to vary the descent distance by dimension. This is useful when laying out rectangles, because it allows one axis to be weighted more heavily than another axis.

[timothypratley]

See issue #333

[timothypratley]

Example usage:

             Descent.dimensionDistance = (x, i, u, v) => {
               var dx = x[i][u] - x[i][v];
               var dedge = i==0 ? (this._nodes[u].width + this._nodes[v].width)/2
                                : (this._nodes[u].height + this._nodes[v].height)/2;
               if (dx > dedge) {
                 dx -= dedge;
               } else if (dx < -dedge) {
                 dx += dedge;
               } else if (dx != 0) {
                 dx /= 10;
               }
               return dx;
             }