DDTPoint minus and add affect both axis

[nathggns]

DDTPoint has this really weird thing that by default arithmetic affects both axis - this is almost never what you want.

We should add a syntax that looks more like this

var point = new DDTPoint(5, 10);
var newPoint = point.add(DDTPoint.X, 5);

newPoint.x
// 10

newPoint.y
// 10

[nathggns]

DDTPoint.X and DDTPoint.Y will simply be constants that point to the DDTAxis enum so we can identify which axis to modify.

If we implement this, we can remove the addToAxis method.