Preserve split type when realoding dataset
Fixes #2167 

Using `eval` is not ideal for security reasons (in web apps I assume), but without it the code would be much more complex IMO.

In terms of style, instead of explicitly importing a private member (`_RelativeInstruction`), we can add these imports at the top of the module:
```python
from . import arrow_reader  # gives us access to ReadInstruction and _RelativeInstruction
from . import splits # gives us access to NamedSplit
```

and then define the `eval` globals as follows:
```python
{**arrow_reader.__dict__, **splits.__dict__}
```


