Visualization helpers

This adds visualization helpers. It adds an interface IVisualization with some methods. Classes can implement methods (conventionally called draw()) that take an instance of IVisualization as a parameter. The class can then use primitives like vertex, line and face on the helper. The helper can be written to an outstream, for instance a file. I've implemented a helper for the PLY and for the OBJ format. I intend to add draw() methods to more classes in the future.

This is the most generic and scalable solution I came up with.