`mean` operator + `SkeletonPair`

[amartinhuertas]

I am positive the following definition

function Gridap.CellData.mean(a::Gridap.Geometry.SkeletonPair{<:Gridap.CellData.CellField})
  Operation(Gridap.CellData._mean)(a.⁺,a.⁻)
end

is missing in Gridap.

I will add it when I have got some time.