review all abstract classes

[Christian-B]

Are they all needed?

Can they be merged or better stacked?

Can they have an empty slots?

[Christian-B]

consider replacing some APi with just Union of types. for example SendsSynapticInputsOverSDRAM

Especially as PopulationNeuronsMachineVertex implements the method exactly the same but is not a SendsSynapticInputsOverSDRAM

[rowleya]

Yes there is a case that if you never ask for instanceof an abstract class, you probably don't need it to be abstract... That is a good example, since the type seems to only exist for typing's sake!