Add bounded kde to default violin plotting style

We should switch the default fit_kde method for the publication-style violin plots to include a bounded kde, especially for parameters that have support at their prior edges. It's quite easy to do this in seaborn by just defining your own fit_kde method, see https://github.com/mwaskom/seaborn/issues/525. This could easily be added to the modified ViolinPlotter class that already exists in PESummary, particularly since there's already a bounded_kde method defined somewhere else in the code 🙂