numba dependency is not pinned; latest version of numba breaks nodevectors

[herbiehuff]

Recently numba removed jitclass from it's module __init__. This breaks the import of jitclass.
numba/numba@4976953

Nodevectors imports jitclass so the dependency needs to be pinned.

Pinning to 0.51.2 fixes the import of nodevectors.

[VHRanger]

We don't use jitclass anymore so I can remove it entirely.

It was tried to map random walks to node names faster but was a pain.

[VHRanger]

This was actually fixed in a previous release of csrgraph.

Please run pip install --upgrade csrgraph to fix it.