Convert PointXYZ to PointXYZRGB to add colour support

Basically, just changed PointXYZ to PointXYZRGB. Now it is now possible to load a point cloud (for example from PLY) and preserve colour information.

At least as far as I can see, all methods working with PointXYZ should work with PointXYZRGB, so I we could just replace the point type completely (unless there is a good reason to stick to PointXYZ an have PointXYZRGB clouds being a separate class).