Allow ipytangle to plug and play with interactive

[tonyfast]

The widgets needs value to automatically update the backbone model; it also needs a description. Can ipytangle be easier to plug into interact and interactive?

[bollwyvl]

Going from interact into tangle is reasonable: once you define an @interact function, you get back a function which has a .widget member (tuple though, yuck).

Here's an example.

In the other direction, adding a value and description is doable, but i guess passing a tangle straight up would mean a pointer to itself so that you still got back the tangle instance...

t = tangle(x=1)
@interact(t=t)
def foo(t):
    print(t.x + 1)

i'd have to figure out how to trigger the change of value on any change on any user-defined traitlet, but that should be okay if it doesn't cause an infinite loop.

As part of #3, i'll end up building widgets to represent all of the traitlets that get created (or reuse ones if a widget value itself is given). that might make this easier, as you could ask for

t = tangle(x=1)
@interact(x=t.widget.x)
def foo(x):
    print(x+1)

[tonyfast]

That example notebook was super helpful. Thanks.

Here is a demo with Bokeh. In this demo, I was unable to find a way to pass the trig function to tangle.