Calculate JL divergence for historical sky maps

Calculate the Jenson-Shannon divergence between baseline and new
sky maps for historical events.

The Jenson-Shannon divergence is the symmetrized Kullback-Leibler
(KL) divergence computed using base 2 logarithms and divided by 2.
It is symmetric and has a value between 0 (for identical
distributions) and 1.

In this patch, we compute the divergence, but it is not yet added
to the MkDocs output.