Backport PR #4147 on branch 1.12.x (perf: welford's algorithm for mean-var aggregation)

[meeseeksmachine]

Backport PR #4147: perf: welford's algorithm for mean-var aggregation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.54%. Comparing base (288e3b9) to head (025e757).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           1.12.x    #4176      +/-   ##
==========================================
- Coverage   78.56%   78.54%   -0.02%     
==========================================
  Files         121      121              
  Lines       12656    12650       -6     
==========================================
- Hits         9943     9936       -7     
- Misses       2713     2714       +1     

Flag	Coverage Δ
hatch-test.low-vers	77.79% <100.00%> (-0.02%)	⬇️
hatch-test.pre	78.43% <100.00%> (-0.02%)	⬇️

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Files with missing lines	Coverage Δ
src/scanpy/get/_aggregated.py	92.66% <100.00%> (-0.65%)	⬇️
src/scanpy/get/_kernels.py	100.00% <ø> (ø)