Rewrite implementation of FairTensor using AxisArrays?

[jiahao]

The functionality for FairTensor seems to be primarily to store labels together with the fairness-confusion tensor. This functionality already exists in https://github.com/JuliaArrays/AxisArrays.jl and I think it would make sense to define FairTensor as an AxisArray.

Also, let's discuss why FairTensor needs to be mutable - there are potential performance penalty issues with the current design.

[ashryaagr]

The LinProgWrapper required the FairTensor to be mutable. But in the commit 62de9fd , I have fixed this requirement and FairTensor (and all wrappers) are now immutable. Thanks for pointing out!

The Pull Request #37 replaces FairTensor with AxisArray. But I think it is better to stick to the struct FairTensor instead of replacing it with AxisArray. The use of FairTensor struct clearly demonstrates a separate entity: FairTensor. I feel that things might get confusing for contributors if we use AxisArray as replacement of FairTensor.