Fix datagrid's grid_type parameter check

[danielkberry]

The assert is checking for ["mean_or_mode", "explicit", and "counterfactual"] when explicit is not a valid parameter, balanced is. This means that the balanced code path is currently not usable (calls to datagrid with grid_type="balanced" will fail).

[vincentarelbundock]

Thanks a ton for spotting this and taking the time to investigate. I really appreciate it!

Merged.