title: The multiplication in xarray
body: <p>We konw that xarray handle data by labels. That is, adding two datasets or multiplying two datasets are operated under the same dimensions and coordinates. However, I have a problem about the shape of the result after xarray multiplication operation.</p>
  <p>The code is shown as follow:</p>
  <pre><code>import xarray as xr
  import numpy as np
  a = xr.DataArray([0,1,2,3],dims=['x'],
                   coords={'x':[10,20,30,40]})
  b = xr.DataArray(np.array([[0,1,2,3],[0,100,200,300]]),
                   dims=['y','x'],
                   coords={'y':['y1','y2'],'x':[10,20,30,40]})
  print(a*b)
  print(b*a)
  </code></pre>
  <p>The first result is</p>
  <pre><code>&lt;xarray.DataArray (x: 4, y: 2)&gt;
  array([[  0,   0],
         [  1, 100],
         [  4, 400],
         [  9, 900]])
  Coordinates:
    * x        (x) int64 10 20 30 40
    * y        (y) &lt;U2 'y1' 'y2'
  </code></pre>
  <p>The second result is</p>
  <pre><code>&lt;xarray.DataArray (y: 2, x: 4)&gt;
  array([[  0,   1,   4,   9],
         [  0, 100, 400, 900]])
  Coordinates:
    * y        (y) &lt;U2 'y1' 'y2'
    * x        (x) int64 10 20 30 40
  </code></pre>
  <p>Actually, the first and second is equal because xarray is label-based, however, why the shape of first result is [4,2] and the second is [2,4]?
  Anyone who can tell me? Thanks!</p>