covariance test success

[Jason Stover]

I just tested the covariance computations with categorical
variables. They computations are correct.

So when do we merge? 

And what to do next? Here is a list of tasks that
stem from having the new covariance.[ch]:

1. Change linreg.c, coefficient.c and regression.q to use the new covariance
routines. 

2. Drop src/data/category.c and covariance-matrix.[ch].

3. Rewrite interaction.c to use covariance.c.

I would prefer to finish a GLM before changing linreg.c too much, but
I'm afraid doing so will just make more work later. Also, linreg.c
will have to be changed to use the new covariance struct anyway, and
doing so without dropping its current behavior of using the entire
data set would make it a lot uglier in the meantime.