matrix package documentation (which we might not end up using at all)
Import MLysy's writeup (XXX contravenes copyright)
freeze the NIW sampling API (in spec only; adapting the routines to actually behave is next)
plot.NIW.marginals

I decided at this point it was not worth the effort to make this subroutine totally generic, and simply
hardcoded that it looks for X and V in its arguments.
I will revisit this choice later.
Ah. My spec was wrong. Use c(d,d,n) instead of c(n,d) matrices.
small writeup nits:

- a space for lemmas, and an unfinished lemma about chol()
- tweak the expected page lengths from our yestermeeting