Some minor fixes for stats.wishart addition.

- fixes to docstrings (references, see also, added in 0.16.0)
- use ``np.linalg.slogdet`` where appropriate
- group tests for multivariate distributions
Hmm, I just checked and for all multivariate distributions adding ``plt.show()`` breaks generating the plots. So I suggest to leave it as is in this PR, and open a new issue for reconciling the different versions of building the docs (and/or a check on minimum Sphinx and numpydoc versions).