Add logmeanexp

[itsdfish]

Hi,

I noticed that R and Python have logmeanexp. I think this would be a simple, yet useful addition. I would be willing to make a PR. The basic function would be

function logmeanexp(X::AbstractArray{<:Number})
    return logsumexp(X) - log(length(X))
end

Some additional logic would be needed to handled the dims keyword.

[tpapp]

Is there a universally accepted definition of logmeanexp? I had to think about what it means.