rates
either a vector of length 2 or a matrix with n rows and 2 columns.
In the vector case, the first element indicates the left-censor rate and the second
element indicates the right-censor rate. Set to NA is there is no censoring.
Interval censored data corresponds to setting both a left-censor rate and right-censor rate.
The default setting indicates a left-censor rate 0.15 with no right censoring.
The vector case handles single censoring and the matrix case is for multiple
censor points. In this case each column indicates the corresponding censoring
for each observation.