ProductSplit uses ProductSection

[jishnub]

Also Improvement in whichproc

on master:

julia> xrange_long, yrange_long, zrange_long = 1:3000, 1:3000, 1:3000;

julia> params_long = (xrange_long, yrange_long, zrange_long);

julia> ps = ProductSplit(params_long, 10, 3);

julia> val = (10,2,901);

julia> @btime whichproc($params_long, $val, 10)
  1.224 μs (14 allocations: 448 bytes)
4

after this PR:

julia> @btime whichproc($params_long, $val, 10)
  357.481 ns (0 allocations: 0 bytes)
4

[codecov]

Codecov Report

Merging #6 (523a0ca) into master (37af17d) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master       #6      +/-   ##
==========================================
- Coverage   99.71%   99.71%   -0.01%     
==========================================
  Files           8        8              
  Lines        1061     1060       -1     
==========================================
- Hits         1058     1057       -1     
  Misses          3        3              

Impacted Files	Coverage Δ
src/errors.jl	100.00% <ø> (ø)
src/productsplit.jl	100.00% <100.00%> (ø)
src/utils.jl	100.00% <100.00%> (ø)

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