Function to query the non-zero index of a OneElement

[jishnub]

This adds a public function to query the index at which a OneElement contains a non-zero value.

julia> O = OneElement(3, (1,2), (4,5))
4×5 OneElement{Int64, 2, Tuple{Int64, Int64}, Tuple{Base.OneTo{Int64}, Base.OneTo{Int64}}}:
 ⋅  3  ⋅  ⋅  ⋅
 ⋅  ⋅  ⋅  ⋅  ⋅
 ⋅  ⋅  ⋅  ⋅  ⋅
 ⋅  ⋅  ⋅  ⋅  ⋅

julia> FillArrays.nzind(O)
CartesianIndex(1, 2)

This complements getindex_value, which returns the stored value, and size, which returns the size. With this, we may obtain all the input arguments without accessing the fields of the struct.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e9077c3) 99.89% compared to head (36c3b37) 99.89%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #345   +/-   ##
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  Coverage   99.89%   99.89%           
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  Files           8        8           
  Lines         934      935    +1     
=======================================
+ Hits          933      934    +1     
  Misses          1        1           

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