Returns the hazard-weighted average of covariate values for subjects who are at-risk at a particular time.
params : ndarray
averages : list of ndarrays
averages[stx][i,:] is a row vector containing the weighted average values (for all the covariates) of at-risk subjects a the i^th largest observed failure time in stratum stx, using the hazard multipliers as weights.
Used to calculate leverages and score residuals.