covariance-matrix.c (is_origin): New function

covariance-matrix.c (get_exact_subscript): New function

covariance-matrix.c (covariance_matrix_insert): Use get_exact_subscript.

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

covariance-matrix.c (covariance_accumulator_to_matrix): Iterate over
the rows and columns of the covariance matrix, and the hash table to
accumulate values for each entry of the matrix.

design-matrix.c (design_matrix_get_n_cols): New function

design-matrix.c (design_matrix_get_n_rows): New function