Fixing the seaborn submodule

This PR introduces some old code from seaborn as a quick fix for the incompatibility with seaborn 0.13 (see issue #302). Some of the imports are also corrected. I would like to stress that this is not a very good solution ; this is intended to be an easy fix to allow using seaborn 0.13 until a better solution is found.

I would like to stress out that this will break again in the feature; we should not be using any private seaborn attributes are those have no guaranteed backwards-compatibility. As far as I can tell the seaborn module in this package was made to allow for using a custom KDE estimator; is this really needed ? Does it need to be made using seaborn ? In any case, I would encourage to migrate it to use the new seaborn.objects interface, which is much more modular and would allow to have a cleaner approach which should hopefully be more resilient (worth noting that it does not currently implement violin plots currently, which is a caveat) ; contributing to seaborn for this kind of stuff is also probably better than keeping this within pesummary in the long run.

Important caveat : the current code works only for seaborn>=0.13 ; if this is an issue, I can add some extra code for backwards compatibility with older seaborn versions.