Create `MOInputHeterotopic` type

[thomasgudjonwright]

Might we want something similar to MOInputIsotopicByFeatures and MOInputIsotopicByOutputs for heterotopic data? I think this would be a good idea since we could then define some convenient helper functionality to extract observations associated to specific outputs, build the inputs, etc...

eg. get_output_input(X::MOInputHeterotopic, p::Int) could return the observations for output p

Originally brought up here.