make 2 utils public

[tlambert03]

nd2.AXIS and nd2.is_supported_file

[codecov]

Codecov Report

Merging #41 (974a70d) into main (41fdc78) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #41      +/-   ##
==========================================
+ Coverage   87.39%   87.40%   +0.01%     
==========================================
  Files          11       11              
  Lines        1174     1175       +1     
==========================================
+ Hits         1026     1027       +1     
  Misses        148      148              

Impacted Files	Coverage Δ
src/nd2/__init__.py	100.00% <100.00%> (ø)
src/nd2/_util.py	90.56% <100.00%> (ø)

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