PFS and OS curves crossing #360
Description
It would be great to have a work around for when PFS and OS curves cross when using heemod for a partitioned survival model. Currently, this causes an error 'Error in compute_counts.eval_part_surv(x = transition, init = init, inflow = inflow): Negative counts in partitioned survival model, at cycles: 1, 2, etc.'
Sometimes, the best fit for the PFS and OS curves will cross. In this case, the progressed disease state occupancy should be calculated as max(0, overall survival - progression free survival).