Things to change before making a breaking release

[thomasgudjonwright]

1. Renaming of multi-output related items, see
   https://github.com/JuliaGaussianProcesses/KernelFunctions.jl/issues/328 and
   https://github.com/JuliaGaussianProcesses/KernelFunctions.jl/issues/332

2. Change the multi output input type definitions to not have abstract types as fields. Change to something like:

struct MOInputIsotopicByFeatures{S,T<:AbstractVector{S}, U<:Integer} <: AbstractVector{Tuple{S,U}}
    x::T
    out_dim::U
end

3. Change MOInputIsotopicByFeatures to MOInputsIsotopicByFeatures and MOInputIsotopicByOutputs to MOInputsIsotopicByOutputs

4. Clean up field names in kernel structs

Add to the list as you see fit :)

[willtebbutt]

As @thomasgudjonwright pointed out, the docstrings for MOInputsBy... need to be added to the docs: https://github.com/JuliaGaussianProcesses/KernelFunctions.jl/pull/353#discussion_r687580200