Oneway: Use covariance matrix and sweep operator

Oneway: Use covariance matrix and sweep operator

Calculate the anova table using the routines from
src/math/covariance.c and lib/linreg/sweep.c instead
of the add hoc method.  This change doesn't remove
all traces of the old method, since the data is still
needed for some subcommands.  This will be the subject
of future changes.