covariance-matrix.c: Added one-pass algorithm to compute covariance

matrices.  New structure covariance_matrix contains the covariance
matrix, related data and member functions. Added different routines to
handle pairwise or listwise deletion of missing values.  Added many
new functions.

glm.q: Changed run_glm to use the new interface to the covariance
matrix. Removed unused function data_pass_one().