coef() function to extract the regression coefficientes under the
MLE approach
vcov() function to compute the variance-covariance matrix associated
with the regression coefficients under the MLE approach
confint() function to compute the 100(1-alpha)% confidence intervals
under the MLE approahc
Bug fixes
yppe() function now works properly if both the number of intervals
and the time grid (n_int and rho) are passed as arguments; the default
time grid is now computed by using all distinct observed failure times
as the endpoints of the intervals