Remove mean method for rotations

[hyrodium]

This PR fixes #91.

The Statistics.mean(::AbstractArray{<:Rotations}) method should return SMatrix. (cf. #91 (comment))
I've just removed the current definition of the method mean(::AbstractArray{<:Rotations}).

Before this PR

julia> using Rotations, Statistics

julia> mean([RotX(i) for i in 1:3])
ERROR: UndefVarError: mean not defined
Stacktrace:
 [1] mean(vec::Vector{RotX{Float64}})
   @ Rotations ~/.julia/dev/Rotations/src/mean.jl:50
 [2] top-level scope
   @ REPL[2]:1

After this PR

julia> using Rotations, Statistics

julia> mean([RotX(i) for i in 1:3])
3×3 StaticArrays.SMatrix{3, 3, Float64, 9} with indices SOneTo(3)×SOneTo(3):
 1.0   0.0        0.0
 0.0  -0.288612  -0.630629
 0.0   0.630629  -0.288612

Note

I think the original method is useful if works fine, but the method should be renamed such as mean_rotation instead of Statistics.mean.
This should be fixed in other PR, and I haven't removed the original file src/mean.jl.

[codecov]

Codecov Report

Merging #177 (829824b) into master (d34316c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #177   +/-   ##
=======================================
  Coverage   86.33%   86.33%           
=======================================
  Files          14       14           
  Lines        1325     1325           
=======================================
  Hits         1144     1144           
  Misses        181      181           

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