title: what is the return value of numpy mean()?
body: <p>I found this problem when calculating array from rasters:</p>
  
  <pre><code>with rasterio.open(file) as ds:   
      arr3d=ds.read()
      arr3d=np.ma.masked_where(arr3d==-32768,arr3d,False)
      list=[]
      for i in range(0,24):
          tmean=arr3d[i,:,:].mean()  
          list.append(tmean)
  </code></pre>
  
  <p>I just wanted to get the list containing 24 mean values, but this code returned the list including each layer of arr3d, its mask layer and mean values.</p>
  
  <pre><code> len(list)=72       
  </code></pre>
  
  <p>But when I tried <code>arr3d[i,:,:].mean()</code>, just retruned a mean value without any array. 
  What is the differce between <code>arr.mean()</code> and <code>np.mean(arr)</code>?</p>