add function to compute NPDEs

Similar to VPC. Compute NPDE based on the generated sim and obs data. So should be a downstream function from VPC, e.g.

```
res <- run_vpc(model = "...", data = ...)
calculate_npde(res)
```
This should output a tibble with columns ID, TIME, DV, PRED, NPDE.

I'm not sure we should add a plotting function to plot these results, we can discuss, but probably better to keep the package clean.