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.