Estimate(s) from the conditional gamma distribution based on given parameters
Arguments
rnd
Vector of quantiles
rate
The rate parameter
shape
The shape parameter
lower_bound
The lower bound to be used (current time)
s_obs
is the survival observed up to lower_bound time,
normally defined from time 0 as 1 - pgamma(q = lower_bound, rate, shape) but may be different if parametrization has changed previously