Turn Neuron.neurotransmitter into an ObjectProperty

[mwatts15]

Currently, Neuron.neurotransmitter is a DatatypeProperty and holds string values. With new Transmitter types, these should hold Transmitter values.

blocked by #286

[clbarnes]

We should also refactor any methods which take a neurotransmitter as an argument, and do neurotransmitter_arg = str(neurotransmitter_arg) if neurotransmitter_arg instanceof Transmitter else neurotransmitter_arg.upper(), so that people can still just type the name if they want.