Revert "Temporarily remove linear-regression prediction material"

This reverts commit f51640f4e4b064782632c9e63682d93c5f7d89c6.
I probably should not remove material from old versions w/out
extremely good reasons, even if they need to be heavily revised.
Revert "Temporarily remove Bayesian material"

This reverts commit 2460007544aed068d48876866b31671ff2834b56.
Ditto.  Shouldn't remove old material unless it's permanent.
Rewrite introduction to linear regression material

This commit is a quick rewrite of the intro materal for linear
regression, mostly changing it so that it uses sentences and
paragraphs (as usual).
fixup intro rewrite
Move homework question from text to problem set
In the process of revising the regression modeling material
Remove material on nonlinear regression

The treatment of splines, local smoothers, etc., was pretty
half-assed.  I'll try to add it back in when it can be
developed.
More work on modeling text
Remove material on proxy variables for now...
Revert "Remove material on nonlinear regression"

This reverts commit 43b8c215595255f18dc1ae8dcf6d5bc63a12a0f7.