Better handling of flat data in numba function and spindles detect

[raphaelvallat]

Closes #85

[codecov-commenter]

Codecov Report

Merging #86 (6875b85) into master (a281d55) will decrease coverage by 0.11%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
- Coverage   90.52%   90.41%   -0.12%     
==========================================
  Files          22       22              
  Lines        2492     2504      +12     
==========================================
+ Hits         2256     2264       +8     
- Misses        236      240       +4     

Impacted Files	Coverage Δ
yasa/numba.py	21.66% <0.00%> (-1.55%)	⬇️
yasa/tests/test_detection.py	100.00% <100.00%> (ø)
yasa/tests/test_numba.py	100.00% <100.00%> (ø)

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