nat.fastcore - Re-implementation of nat core functions in Rust

A re-implementation of core 'nat' (Neuroanatomy Toolbox) functions in Rust for speed. Provides fast skeleton/tree (DAG) operations (geodesic distances, Strahler order, segment generation, twig pruning, synapse flow centrality, node classification, skeleton healing), mesh connected components, NBLAST/synNBLAST neuron similarity scoring, CMTK and Elastix spatial transforms (applying a registration to points without needing CMTK or Elastix installed), and landmark-based warps (thin-plate spline, moving least squares), exposed to R via extendr.