update package to the pkg3 format

[tlnagy]

No description provided.

[codecov-io]

Codecov Report

Merging #8 into master will increase coverage by 5.25%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master       #8      +/-   ##
==========================================
+ Coverage   62.63%   67.88%   +5.25%     
==========================================
  Files           3        3              
  Lines          91      109      +18     
==========================================
+ Hits           57       74      +17     
- Misses         34       35       +1

Impacted Files	Coverage Δ
src/utils.jl	100% <0%> (ø)	⬆️
src/Coulter.jl	86.95% <0%> (+1.95%)	⬆️
src/analysis.jl	52.94% <0%> (+8.29%)	⬆️

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