Add Rotations.params methods for RotationGenerator

[hyrodium]

This PR fixes #213.

Before this PR

julia> using Rotations

julia> RotationVecGenerator(1,2,3)
3×3 RotationVecGenerator{Int64} with indices SOneTo(3)×SOneTo(3)(1, 2, 3):
  0  -3   2
  3   0  -1
 -2   1   0

julia> Rotations.params(RotationVecGenerator(1,2,3))
ERROR: MethodError: no method matching params(::RotationVecGenerator{Int64})
Closest candidates are:
  params(::RotZY) at ~/.julia/dev/Rotations/src/euler_types.jl:259
  params(::AngleAxis) at ~/.julia/dev/Rotations/src/angleaxis_types.jl:40
  params(::RotY) at ~/.julia/dev/Rotations/src/euler_types.jl:37
  ...
Stacktrace:
 [1] top-level scope
   @ REPL[4]:1

After this PR

julia> using Rotations

julia> RotationVecGenerator(1,2,3)
3×3 RotationVecGenerator{Int64} with indices SOneTo(3)×SOneTo(3)(1, 2, 3):
  0  -3   2
  3   0  -1
 -2   1   0

julia> Rotations.params(RotationVecGenerator(1,2,3))
3-element StaticArrays.SVector{3, Int64} with indices SOneTo(3):
 1
 2
 3

[codecov]

Codecov Report

Merging #216 (482a07e) into master (754769c) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #216      +/-   ##
==========================================
+ Coverage   88.13%   88.14%   +0.01%     
==========================================
  Files          15       15              
  Lines        1601     1603       +2     
==========================================
+ Hits         1411     1413       +2     
  Misses        190      190              

Impacted Files	Coverage Δ
src/rotation_generator.jl	95.90% <100.00%> (+0.06%)	⬆️

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