covariance_accumulate: New function for one-pass computation of the

covariance_accumulate: New function for one-pass computation of the
covariance matrix.

Added hash table to temporarily store the elements that will be in the
covariance matrix. The hash consists of the new data structure
covariance_accumulator. New functions hash_numeric_alpha,
covariance_accumulator_compare, match_nodes,
covariance_accumulator_free, covariance_hsh_create,
covariance_accumulator_hash, to assist with the hashing.

update_product: New function.

covariance_accumulator_to_matrix: New function to put hash elements
into the covariance matrix.

get_center: New function to center a value prior to adding it to
the covariance matrix.

covariance_matrix_insert: New function to add a value to the
covariance matrix.