Current SBT implementation is poorly localized

[olgabot]

As mentioned here, the current implementation of SBTs is poorly localized. When building a nearest neighbor graph on the current SBTs, which assumes that leaves sharing parents are more similar than leaves not sharing neighbors, one gets the right side of the nearest neighbor graph, instead of the correct one on the left.

Thanks @phoenixAja for getting me to finally write up the issue :)

[olgabot]

This could be rust-y?

[olgabot]

Could be related to: #545 (allsome/split-sbt implementations)