Fix zi_helpers.get_value return type hint

There appears to be an issue with the typing of zi_helpers.get_value:

def get_value(instrument: base.ZIBaseInstrument, node: str) -> str:

However, in monitor_acquisition_resolver:

    results_i = zi_helpers.get_value(uhfqa, node_i)
    results_q = zi_helpers.get_value(uhfqa, node_q)
    return results_i + 1j * results_q

And in result_acquisition_resolver

    vals_node0 = zi_helpers.get_value(uhfqa, result_nodes[0])
    vals_node1 = zi_helpers.get_value(uhfqa, result_nodes[1])

    # the ZI API keeps the contributions of both weight functions separate
    # here we combine them so they correspond to the I and Q components.
    vals_i = vals_node0.real + vals_node0.imag
    vals_q = vals_node1.real + vals_node1.imag

    results = vals_i + 1j * vals_q

Would it be the case that the output of the get_value should be typed as np.array? Or?