covariance-matrix.c: Add matrices to store valid sample sizes and

covariance-matrix.c: Add matrices to store valid sample sizes and
products of means in struct covariance_matrix.

covariance-matrix.c (update_ssize): New function

covariance-matrix.c (covariance_accumulator_to_matrix): Store valid
sample sizes and products of means, then use them to compute the
entries of the covariance matrix after passing through the
hash. Return void.