Consistency across external functions

[mr-rooftop]

1. Some external functions have return types, others do not.

2. : In transfer you can provide _data which then gets put into the Event:

   def transfer(self, _to: Address, _value: int, _data: bytes = None):
   ->
   self.Transfer(_from, _to, _value, _data)
   Logger.debug(f'Transfer({_from}, {_to}, {_value}, {_data})', TAG)

   In Join there's no _data:

   def join(self) -> None:
   ->
   self.Join(sender, value)

   And what about leave?