Extend copytrito! for a sparse source

[jishnub]

julia> S = sprand(100, 100, 0.01);

julia> M = zeros(size(S));

julia> @btime copytrito!($M, $S, 'U');
  20.779 μs (0 allocations: 0 bytes) # main
  2.050 μs (0 allocations: 0 bytes) # this PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.10%. Comparing base (33fbc75) to head (16076cd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #533      +/-   ##
==========================================
+ Coverage   76.42%   84.10%   +7.67%     
==========================================
  Files          12       12              
  Lines        8969     9020      +51     
==========================================
+ Hits         6855     7586     +731     
+ Misses       2114     1434     -680     

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[ViralBShah]

Maybe just make it M[row,col] = nz[i]?

[ViralBShah]

Looks straightforward. Please merge when ready.