Optimize Pixel Tree

[smcguire-cmu]

Currently, the pixel tree uses individual python objects for its nodes. For larger trees, this means it performs pretty slowly. By changing the tree structure to instead use a simpler object like a tuple, this could improve performance. This would mean a slight change to the pixel tree interface, but given that most of our interactions with the tree come from performing a pixel alignment and working with that, I don't think this will require big changes across the code base.

Alternatively, the tree could be written in something like C or Rust and wrapped for the best performance.