Simulate multi-state data

[dincerti]

A function based on the existing IndivCtstmTrans$sim_disease() method could be created so that users can simulate example multi-state data. The difference from the existing output is that there should be multiple rows for each possible transition from a given state (only 1 is needed for simulating outcomes) and the states that a patient does not transition to should be flagged as censored.

This function would be helpful for performing Monte Carlo simulations and assessing the performance of statistical methods for mulit-state data.