Add promote rule for the type of raw probabilities

[ablaom]

using CategoricalDistributions
d1 = UnivariateFinite([0.2, 0.3])
d2 = UnivariateFinite([Float32(0.2), Float32(0.3)]);
v = [d1, d2];
julia> typeof.(v)
2-element Vector{DataType}:
 UnivariateFinite{Multiclass{2}, String, UInt8, Float64}
 UnivariateFinite{Multiclass{2}, String, UInt8, Float32}
julia> eltype(v)
UnivariateFinite{Multiclass{2}, String, UInt8}

What the last line in the code snippet above "should" be is UnivariateFinite{Multiclass{2}, String, UInt8, Float64}, which a promote_rule "lifting" promotion in P will do, right?

[ablaom]

This will resolve JuliaAI/MLJBase.jl#835 .