Create a RiemannSurface constructor function

[cswiercz]

It's really messy performing all of the required initialization in RiemannSurface.__init__, particularly when managing all of the dependencies on RiemannSurfacePathFactory. Besides, if the path factory object bugs out for some reason then we might be left with an uninitialized object which is really poor programming style.

Instead, create a function RiemannSurface which returns a RiemannSurfaceGeneric object. (This also sets up the possibility of a RiemannSurfaceHyperelliptic object which simplifies certain computations.) The RiemannSurfaceGeneric object will, in turn, be initialized with a PathFactory object which, I believe, only needs to depend on the underlying curve.

This is an opportunity for a better design as well as an opportunity to start working on a Voronoi-based path factory.