I think this should allow `param_dim` * `npars` sizes, e.g.:
```
G = models.builtin_models.Gaussian1DModel(1,1,1)
G.parameters = [1,2,1,2,1,2]
```
which would be equivalent to
```
G = models.builtin_models.Gaussian1DModel([1,2],[1,2],[1,2])
```