covariance branch update

[Jason Stover]

I just pushed a rewrite of linreg.c and regression.q
to the covariance branch. It uses the code in covariance.c.
Please let me know if looks OK. It passed make check.

If it does look OK, then I should merge this to master
and delete covariance-matrix.[ch], coefficient.[ch],
and design-matrix.[ch].

The next step will be to make covariance.[ch] able
to handle interactions. That could be difficult.

-Jason