Sync #2: Switch .lyx -> .tex

You should be able to use InvWishart/docs/writeup/mathematics.tex now. Add to it as you see fit. When it gets too large, we can split it to subfiles.

I played with Rcpp (thanks for doing the legwork on that).  For completeness, I recorded what you discovered in test.R to go with test.cpp.

And there's some scrap diffs in here that I committed since I sent the previous pull request.