Remove AllenSDK dependency

[adamltyson]

AllenSDK cannot be installed with Python 3.12. As good as this package is, it seems that we can't always rely on it to be maintained satisfactorily. I also think it's too large and complex for us to be able to contribute meaningfully to it's maintenance (although I could be wrong).

To get around this, we should remove the dependency and fetch the data directly (e.g. via REST API). We could also fetch the streamlines data this way, so we no longer rely on neuromorpho.org.

cc @IgorTatarnikov