title: Create empty 3D numpy array and append it with each new given value
body: <p>I feel like there has to be an easy solution to this, but I'm not too familiar with Numpy yet and couldn't find anything in the library fitting my needs. I feel like this is an obvious thing, but I couldn't find any similar question regarding this on StackOverflow.  </p>
  
  <p>Basically, I want to create a 3-dimensional <strong>empty</strong> array. I tried it like this: <code>matrix = np.array(None, ndmin=3)</code>. And now every time I have a new variable I want to append a dimension of the matrix I chose with the new value of the variable. How can I do that? I played around with <code>np.append()</code> and <code>np.insert()</code> but I couldn't come to a solution.</p>