An object of class qalys returned from methods
$sim_qalys() in model classes that store simulated
quality-adjusted life-years (QALYs).
A qalys object inherits from data.table and contains
the following columns:
A random sample from the PSA.
The treatment strategy ID.
The patient ID.
The subgroup ID.
The health state ID.
The rate used to discount QALYs.
A single category always equal to "qalys".
The simulated values of QALYs.
If the argument lys = TRUE, then the data.table also contains a column
lys containing simulated life-years.