Add dimension specification for mode()

[matago]

I am wondering if it would be possible to implement dimension specification for mode() in a similar fashion to mean() and sum(). mode(A,1) would provide column modes and mode(A,2) row modes across Array A

[nalimilan]

Shouldn't be too hard, but I wonder whether we should go into that direction: then we'll eventually have to implement this for all reductions. Maybe we should keep this limited to sum and mean, and use reducedim(f, A, dims) for other operations?

[nilshg]

Would this be covered by #77 and thus the fact that we now have eachslice?