A pair of functions for calculating mean residual life (MRL)
, median residual life, and percentile residual life using the outputs of
either the 'flexsurv' package or parameters
provided by the user. Input information about the distribution, the given
'life' value, the percentile, and the type of residual life,
and the function will return your desired values. For the 'flexsurv' option,
the function allows the user to input their own data for making predictions.
This function is based on Jackson (2016)