Added accessors to quaternion elements

[korken89]

Hi,

A small improvement for accessing quaternion elements.

[codecov-commenter]

Codecov Report

Merging #89 (7df22b1) into main (2458446) will decrease coverage by 0.51%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main      #89      +/-   ##
==========================================
- Coverage   57.42%   56.90%   -0.52%     
==========================================
  Files          38       38              
  Lines         876      884       +8     
==========================================
  Hits          503      503              
- Misses        373      381       +8     

Impacted Files	Coverage Δ
src/quaternion.rs	0.00% <0.00%> (ø)

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[tarcieri]

Thank you!