Add pyx file again

[hildensia]

Was removed during a PR. Is there a good way to keep cython and python in sync. I'm not sure if I prefer one over the other (python is better for debugging, cython is faster).

[mathDR]

I haven't found a good way to keep them in sync. I think right now the examples are plenty fast. In any case, I added the original cy_offline.pyx and cy_offline_changepoint_detection.py files back to the repository.

[mathDR]

Also, its not clear that the speedup gains are optimized. If the new multivariate likelihoods are to by cythonized, then outer(), slogdet(), multigammaln() and gammaln() should all be cythonized. Thoughts?

[hildensia]

It is probably not really worthwhile using cython right now. I just tried it for an experiment, but it wasn't really speeding things up a lot. For my part it can be removed.

[mathDR]

Should we close this issue then?

[hildensia]

Yepp. You're right.