Grant @grantmcdermott, this is a follow-up to #270 where I did the following in three separate commits:

• Made joint.bw = "owm" the default in type_density() and enhanced the documentation. In particular, I added a new section explaining the practical considerations for choosing the different joint.bw alternatives. I wrote this mostly so that we do not forget about it but feel free to shorten it (or remove it entirely) if you feel that this is too detailed.
• Update the type_ridge() documentation to the changes in the type_density() documentation.
• Use joint.bw = "mean" instead of "owm". I find the latter too opaque and it over-emphasizes the weighting. I find "mean" much more straightforward and the weighting is natural (to me) and well-explained in the documentation. If you feel strongly about not using "mean", feel free to revert this commit.

If you keep the last commit, you way want to further adjust the tests. I have changed their code so that they run successfully but kept "owm" in the annotations so that the snapshots don't need updating, yet.

Closes #290