vignette does not use correct V

[stephens999]

in https://stephenslab.github.io/udr/articles/udr_intro.html
the initial run uses the "default" V=Identity whereas the true V is
different. That's not something we want to encourage I think....

[stephens999]

building on this, probably we need to update defaults etc in light of new work.

I suggest we require the user to specify a number of components rather
than trying to provide any kind of default for that. And that
we don't create rank1 or fixed covariances without them explicitly asking.
So the vignette first example might look something like:

fit0 <- ud_init(X,n_unconstrained = 10,V = V)

and this would fit a mixture with 10 unconstrained components.

[yunqiyang0215]

Yes. The default has to be changed, as we don't encourage the use of rank-1 Us. How about canonical Us?

[stephens999]

on canonical Us, we don't discourage, but they should not be included as default. They are an "additional feature".

[yunqiyang0215]

I updated the udr_intro vignette and changed the default setting to n_rank = 0. I think this issue can be closed now.