covariance-matrix.c: (is_covariance_contributor) Removed unnecessary recursion.

covariance-matrix.c: (is_covariance_contributor) Removed unnecessary recursion.

covariance-matrix.c (is_origin): Do not count a numeric variable as
the origin. Call is_origin only for categorical variables.

covariance-matrix.c (ordered_match_nodes): New function.

covariance-matrix.c (match_nodes): Use bitwise operators and
ordered_match_nodes for hashing instead of so many ifs.

covariance-matrix.c (hash_numeric_alpha): Use hash_value_short to hash
combinations of numeric and categorical variables.

covariance-matrix.c (get_value_from_subscript): New function to match
values and columns in a design matrix.

covariance-matrix.c: Removed unused design matrix struct sums from
struct covariance matrix. To get the means, use struct moments
instead.

covariance-matrix.c (covariance_accumulator_to_matrix): Use
design_matrix_set_element instead of covariance_matrix_insert. Drop
unused function covariance_matrix_insert.

covariance-matrix.c (covariance_accumulator_hash): Use hash_bytes instead of hsh_hash_bytes.