add Slerp for 3D rotations

[bicycle1885]

This PR adds slerp for 3D rotations. The method for QuatRotations is the canonical implementation and methods for other rotation types are implemented via this canonical one.

I don't know which representation it should return for arguments with mixed types. The proposed one returns typeof(r1) for slerp(r1, r2, t).

Closes #214.

[codecov[bot]]

Codecov Report

Merging #252 (6b49efb) into master (2f9be71) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #252      +/-   ##
==========================================
+ Coverage   88.82%   88.84%   +0.02%     
==========================================
  Files          16       16              
  Lines        1602     1605       +3     
==========================================
+ Hits         1423     1426       +3     
  Misses        179      179              

Impacted Files	Coverage Δ
src/euler_types.jl	92.27% <100.00%> (+0.01%)	:arrow_up:
src/unitquaternion.jl	97.56% <100.00%> (+0.02%)	:arrow_up:

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[bicycle1885]

Thank you for reviewing. I've update the PR following your advice. Can you look at the changes and give me feedback again?

[bicycle1885]

Thank you!

[c42f]

I haven't been active here lately, but I thought I'd pop in and say thanks @hyrodium for doing a great job maintaining this package. It's amazing to have people like you step in and care for code I once worked on, but don't have time for right now. Thanks :heart:

(Also thanks @bicycle1885 for the specific contribution here, this looks nice :-) )