Hard optimizations and much more rigorous testing for correlation sum

[Datseris]

• Added brutally accurate tests that really test the thing to oblivion
• Added threading that makes everyting 3-4x faster
• Removed allocations here and there
• Better code organization

[codecov-commenter]

Codecov Report

Merging #11 (a0a55c7) into main (e93dbd1) will decrease coverage by 28.24%. The diff coverage is 27.73%.

@@             Coverage Diff             @@
##             main      #11       +/-   ##
===========================================
- Coverage   90.47%   62.24%   -28.24%     
===========================================
  Files           9       10        +1     
  Lines         483      686      +203     
===========================================
- Hits          437      427       -10     
- Misses         46      259      +213     

Impacted Files	Coverage Δ
...rsum_based/correlationsum_boxassisted_different.jl	0.00% <0.00%> (ø)
src/corrsum_based/correlationsum_vanilla.jl	53.69% <36.98%> (-38.62%)	:arrow_down:
src/corrsum_based/correlationsum_boxassisted.jl	84.11% <100.00%> (+0.58%)	:arrow_up:
src/corrsum_based/correlationsum_fixedmass.jl	100.00% <100.00%> (ø)

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[Datseris]

What the hell is this report saying, I added like 100 more tests and the coverage drops by 20%? Get out of here!