simVpc needs to more closely match the estimated model

[mattfidler]

There is a case where the vpcSim does not match the estimated model as well as using the $foceiModel$predOnly to do the vpc. This means the vpcSim needs to be updated to more closely match the focei and other sorts of models...

[mattfidler]

This also probably means assigning the simulation model to fit$ui$.simulationModel in the appropriate context

See below:

https://github.com/nlmixr2/rxode2/blob/efcee1c8412d0ce098dc760237f317ae9b363e76/R/err-sim.R#L225-L233

[mattfidler]

Also see:

nlmixr2est/R/simulate.R

Line 55 in 7881587

.getSimModel <- function(obj, hideIpred=FALSE, tad=TRUE) {