Potential improvement: work with dicts

[vankesteren]

ArrayMargins, ArrayFactors are currently objects with two slots:

• the margins or factors
• DimIndices

this is pretty similar to just having a dictionary (with some extra sauce). Potentially, dictionaries are the nicer data structure here?

struct ArrayFactors{T} <: AbstractDict{K,V} where K = Vector{Vector{Int}} where V = Vector{<:AbstractArray{T}}

look here: https://github.com/JuliaLang/julia/blob/master/base/dict.jl#L57-L79