Add get_[inputs|outputs]_len to CNNNetwork

[abrown]

This change is an adaptation of #33 that allows for the same kind of functionality. It also adds tests checking the newly-added functions. With this in place, users will be able to set input layouts with something like:

for i in 0..network.get_inputs_len() {
  let name = network.get_input_name(i)?;
  network.set_input_layout(&name, Layout::NHWC)?;
}