add function to make VPC for model + dataset

We can easily re-use the components from `run_eval()` to make a function `run_vpc()` or something, that creates VPC data using a PKPDsim model, and a NONMEM dataset, without the use of NONMEM. You'd only do

```
vpc_dat <- create_vpc_data(
    "pk_vanco_thomson",
    data
)
```

and then plot:

```
library(vpc)
vpc(sim = vpc_dat$sim, obs = vpc_dat$obs)
```