Fix principal_value method

[hyrodium]

This PR fixes #179.

julia> principal_value(RodriguesParam(1,2,3))
3×3 RodriguesParam{Int64} with indices SOneTo(3)×SOneTo(3)(1, 2, 3):
 -0.733333  -0.133333  0.666667
  0.666667  -0.333333  0.666667
  0.133333   0.933333  0.333333

julia> principal_value(Angle2d(5.0))
2×2 Angle2d{Float64} with indices SOneTo(2)×SOneTo(2)(-1.28319):
  0.283662  0.958924
 -0.958924  0.283662

[codecov]

Codecov Report

Merging #180 (83244fe) into master (20a730f) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #180      +/-   ##
==========================================
+ Coverage   86.33%   86.41%   +0.07%     
==========================================
  Files          14       14              
  Lines        1325     1325              
==========================================
+ Hits         1144     1145       +1     
+ Misses        181      180       -1     

Impacted Files	Coverage Δ
src/principal_value.jl	100.00% <100.00%> (+4.16%)	⬆️

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