Add params methods for RotationGenerator

[hyrodium]

I was forgetting to add params methods in #203.

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