Allow weights in cut(x, ngroups)

[greimel]

[Branches from #202, there is only one new commit]

Adds a method

cut(x::AbstractArray, w::AbstractWeights, ngroups::Integer;
  labels) =
  cut(x, StatsBase.quantile(x, w, (1:ngroups-1)/ngroups); extend=true, labels=labels)

Adds StatsBase as a dependency.

Is this something that you want or should that live in a separate package?

[nalimilan]

Thanks. Unfortunately, I don't think it's a good idea to depend on StatsBase, as it makes loading the package even slower. Hopefully, JuliaLang/julia#31395 will allow doing this with Statistics in Julia 1.4.

[nalimilan]

Superseded by #423.