CrossValidation type

[tpoisot]

Currently cross-validation is returned as a named tuple with validation and training. It would be much nicer to do

mutable struct CrossValidation
   validation::Vector{ConfusionMatrix}
   training::Vector{ConfusionMatrix}
end

This is fully backward compatible but we can have much nicer interface with e.g. PrettyTables and maybe a plotting method?

[tpoisot]

Would be so much nicer to have by the CSEE workshop