overload getindex to select particle type via Collection field

[elehcim]

No description provided.

[codecov]

Codecov Report

Merging #22 (4de76f8) into master (7101faa) will increase coverage by 0.13%.
The diff coverage is 100.00%.

❗ Current head 4de76f8 differs from pull request most recent head c7a9461. Consider uploading reports for the commit c7a9461 to get more accurate results

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
+ Coverage   85.83%   85.97%   +0.13%     
==========================================
  Files          13       13              
  Lines         713      720       +7     
==========================================
+ Hits          612      619       +7     
  Misses        101      101              

Impacted Files	Coverage Δ
src/Unit.jl	98.52% <ø> (ø)
src/PVector.jl	99.09% <100.00%> (+0.02%)	⬆️
src/PhysicalParticle.jl	75.40% <100.00%> (+1.72%)	⬆️

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