Quaternion to Euler Angles conversion

[korken89]

Hi,

I added the bog-standard Quaternion -> Euler angles conversion (https://en.wikipedia.org/wiki/Conversion_between_quaternions_and_Euler_angles) Not sure how "unity" constraints are viewed in this library, so I went with the simplest approach.

[codecov-commenter]

Codecov Report

Merging #90 (5aab6d1) into main (bbee5cf) will decrease coverage by 0.44%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main      #90      +/-   ##
==========================================
- Coverage   56.90%   56.45%   -0.45%     
==========================================
  Files          38       38              
  Lines         884      891       +7     
==========================================
  Hits          503      503              
- Misses        381      388       +7     

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

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[tarcieri]

Thanks!