Oxidize celltree

[Hofer-Julian]

Expected benefits:

• memory safety
• thread safety
• helpful error messages instead of segfaults (e.g. with out-of-bound access)

Steps:

• migrate one numba function that is not called by other numba functions
• rely on std containers if possible (Vec)
• benchmark
• evaluate debugging experience

[Hofer-Julian]

Conclusions:

• everything related to the std worked smoothly and as expected
• dealing with Rust + ndarray + pyO3 + python-numpy involves jumping though some hoops (repetitive, not too bad)
• Rust adds verbosity to the code (random &, *, and lifetime parameters ' decreasing readibility)
• Error messages related to ndarray are often next to useless
• We couldn't get a specific use case to work (zip two arrays of different dimensions in parallel), but it is unclear whether:
  • It can work, but we missed documentation
  • It doesn't work and rayon-ndarray integration has to be improved
  • Either way, we found that we spend a lot of time fighting the Rust compiler and it didn't give us a lot of useful hints.