Consider allowing linking parameters by passing by reference

[philippjfr]

Consider having two Parameterized classes:

class A(param.Parameterized):

    a_value = param.Number(default=0)

class B(param.Parameterized):

    b_value = param.Number(default=0)

I think it might be nice to be able to link parameters on these classes simple by passing the instance parameter, e.g.:

a = A()
b = B(b_value=a.param.a_value)

Parameter b_value on the B instance would now track the value of a_value on the A instance.

[poplarShift]

Because others might be interested, here is how I'm doing it currently:

def propagate_param(origin, origin_param, target, target_param):
    def callback(*events):
        for event in events:
            setattr(target, target_param, event.new)

    watcher = origin.param.watch(callback, [origin_param], onlychanged=False)
    origin.param.trigger(origin_param)
    return watcher

and then

a = A()
b = B()
propagate_param(a, 'a_value', b, 'b_value')

[Jhsmit]

Is there a way of doing this bidirectionally?

[philippjfr]

Implemented in #843