top level __all__ = [] prevents star imports

Currently, vetiver has this line in its top level `__init__.py`:

```
__all__ = []
```

This means that if someone uses a star import, nothing will be imported:

```python
# nothing gets imported
from vetiver import *
```

I wonder if we should change the `__all__` to either specify all top-level imported functions and classes, or remove `__all__`?