Improved response argument support

[aatmdelissen]

Basically there are two issues remaining with response arguments:

1. Modules don't support keyword arguments in the response(), but only positional arguments. This is due to the inconvenience of constructing a dictionary when returning the sensitivities. If a dict would be returned, this should be possible.

2. It is a bit cumbersome to obtain the values passed to the response function, especially for optional arguments not passed by the user. No signal exists for these inputs and when using get_input_states, only the inputs defined by sig_in are returned. Workaround for now:

class MyModule(pym.Module):
    default_b: float = 1.0
    default_c: float = 1/np.sqrt(2)

    def __call__(self, a, b=None, c=None):
        if b is None:
            b = self.default_b
        if cis None:
            c = self.default_c
       ...

    def _sensitivity(self, dy):
        b, c = self.default_b, self.default_c
        if self.n_in == 1:
            a= self.get_input_states()
        elif self.n_in == 2:
            a, b = self.get_input_states()
        elif self.n_in == 3:
            a, b, c = self.get_input_states()
        else:
            raise ValueError("Wrong number of inputs")
        ....