Remove old riemannsurface module.
Resolve issues with RSP.get_y().

Some segment indexing issues were occuring with RiemannSurfacePath
objects and their getters.
Allow the addition of Riemann surface paths.

This required exposing the underlying RiemannSurface,
starting place, and segment list as Python-accessible
properties. This doesn't seem to cause a performance loss.
Remove obsolete code from riemann_surface.

Artifacts from the pre-refactor days were left in
riemann_surface.pyx. This commit deletes said artifacts.
Include necessary cython files for Riemann theta.

Forgot to add a file to the Cython extensions list. This
should fix the strange errors I've been seeing.