Allow VPC with new data

[billdenney]

On occasion, I want to perform a VPC with new data (see certara/tidyvpc#35 (comment)). The current vpcSim() interface does not allow passing in data (when I tried adding events = newdata, I got an error of duplicated argument names).

I think that it would be useful to allow a VPC with new data for estimation of predictive power for a model in a new dataset. For similarity to other functions, I'd suggest that adding either a data argument (similar to nlmixr()) or adding a newdata argument (similar to predict()) would be the preferred interface. If not given, then the current behavior would be used.