add private helper function for redundant task of preparing newton inputs

[mikofski]

problem
there is WET code in singlediode_methods where the inputs to i_from_v and v_from_i are manipulated for the newton method

solution
according to this comment here's a nice snippet:

def _prepare_newton_inputs(i_or_v, args, v0):
        size, shape = _get_size_and_shape((i_or_v,) + args)
        if size > 1:
            args = [np.asarray(arg) for arg in args]
        # newton uses initial guess for the output shape
        # copy v0 to a new array and broadcast it to the shape of max size
        if shape is not None:
            v0 = np.broadcast_to(v0, shape).copy()
        else:
            v0 = voc_est
    return args, v0

alternatives
do nothing

Additional context
see #409