Need to update neurovault file downloading with new nilearn API

[ejolly]

@ljchang Separate from the issue we were having about hosted datasets on neurovault going missing, currently we can't build our docs because nilearn has changed their API and removed _get_dataset_dir and _fetch_file both of which are used within the datasets.py module.

Can you take a look at this since you wrote it originally? Maybe we need to change how we're handling this or deprecate loading from neurovault altogether given the other issue? Currently this is blocking our ability to deploy new documentation or even test building it locally

[kaiwang0112006]

To quickly use the tools, I reinstalled nilearn as
pip install nilearn==0.10.0