generalizes flux

[ogauthe]

This PR generalizes flux, to be used on ranges and AbstractArray. It also defines UndefinedFlux and checkflux and add tests.

Fix #28

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.37%. Comparing base (d5cc938) to head (fd8f56a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #44      +/-   ##
==========================================
+ Coverage   94.32%   94.37%   +0.04%     
==========================================
  Files          20       20              
  Lines         776      782       +6     
==========================================
+ Hits          732      738       +6     
  Misses         44       44              

Flag	Coverage Δ
docs	0.00% <0.00%> (ø)

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[mtfishman]

Looks good, thanks!