Better function output support

[cphyc]

Right now, the following function

@register_node
def create_tuple() -> Tuple[int, str]:
    return (1, "foo")

allows to create nodes with no inputs and one output with type Tuple[int, str]. It would be great to support two outputs instead, with types int and str respectively. Even better would be to allow Tuple[int, str] to plug into int, str or Tuple[int, str] sockets. In the first two cases, a new output socket would be guessed with type str or int.

This should be enough to support only Tuple outputs rather than List, or more complex types with variable number of elements.