Match behavior between vanilla and fastcore generation of skeleton segments

[schlegelp]

Addresses issue flagged in #179:

Previously, the vanilla implementation of _generate_segments would ignore isolated nodes. That's because it walks from each leaf to the correspond root, and isolated nodes are treated as roots rather than a root that is also a leaf. The implementation of this in navis-fastcore already returns segments containing only a single node.

With this PR, the vanilla implementation of _generate_segments will:

• return single node segments
• return a list of arrays (previously a list of lists)