Rename book again

"Core Econometrics Textbook" is ugly and pretentious; "Core
Econometrics Notes" seems better.  Better to get it right now
than after anyone starts to pay attention"
Merge finite sample documents

The finite-sample documents should be streamlined and shortened
so that they can go in the same "part."  This commit starts that
process by putting them in the same file.
Remove duplicate copies of tufte-latex class

It seems a little silly to have our own copy of the tufte latex
files in the repo.  So this commit removes them.  If necessary,
they can be added back in the future.