Update to pandas 0.23

Created by: Zaharid

So I don't like pandas. Updates are way more intrusive than any other dependency we have. Yet at some point we have to byte the bullet or we end up in a dependency hell.

@eslade can you please test that the alpha_s runcards work as before? Note that alpha_s used to only work with pandas 0.20 and with this patch it only works with pandas 0.23 (and not with 0.20). It wouldn't surprise me that we get some wrong result here and there.

@RosalynLP Can you please make sure I didn't break anything in the theory covariance file?