Get rid of inefficient findnz

[gdalle]

Replace every occurrence of I, J, _ = findnz(S) with the non-allocating iteration:

rv = rowvals(S)
for j in axes(S, 2)
    for k in nzrange(S, j)
        i = rv[k]
        # do stuff
    end
end

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (e26edfe) to head (f30f5be).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #111   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines          755       764    +9     
=========================================
+ Hits           755       764    +9     

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