to_numpy should have strict kwarg and warn rather than err

Converting datashapes with missing types to numpy dtypes raises an error. This is appropriate because numpy dtypes don't support missing values. It might be nice to control this behavior with a `strict` keyword that allows lowering instead to the non-missing equivalent, e.g.

```
?int32 -> np.int32
```

Perhaps we should raise a warning rather than an error in this case?

What should be default behavior?