Conditional quantile function for normal distribution
Arguments
- rnd
- Vector of quantiles 
- mean
- The mean parameter 
- sd
- The sd 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 - pnorm(q = lower_bound, mean, sd) but may be different if parametrization has changed previously 
