title: Add numpy array as variable to Xarray Dataset
body: <p>This seems like a very basic operation, but I can't figure out how to do it using the xarray documentation.</p>
  
  <p>I have an xarray DataSet: </p>
  
  <pre><code>dss
  &lt;xarray.DataArray (y: 1000, x: 1334)&gt;
  dask.array&lt;shape=(1000, 1334), dtype=uint8, chunksize=(222, 58)&gt;
  Coordinates:
      band     int32 1
    * y        (y) float64 2.218e+06 2.218e+06 2.218e+06 2.218e+06 2.218e+06 ...
    * x        (x) float64 1.891e+06 1.891e+06 1.891e+06 1.891e+06 1.891e+06 ...
  Attributes:
      transform:   (30.0, 0.0, -2493045.0, 0.0, -30.0, 3310005.0, 0.0, 0.0, 1.0)
      crs:         +ellps=GRS80 +lat_0=23 +lat_1=29.5 +lat_2=45.5 +lon_0=-96 +n...
      res:         (30.0, 30.0)
      is_tiled:    1
      nodatavals:  (nan,)
  </code></pre>
  
  <p>and a numpy array with the correct dimensions:</p>
  
  <pre><code>print(np.shape(nmap))
  (1000, 1334)
  nmap
  array([[ 0.15,  0.1 ,  0.15, ...,  0.05,  0.05,  0.02],
         [ 0.15,  0.1 ,  0.05, ...,  0.05,  0.05,  0.05],
         [ 0.1 ,  0.15,  0.15, ...,  0.05,  0.05,  0.02],
         ..., 
         [ 0.02,  0.02,  0.02, ...,  0.02,  0.02,  0.02],
         [ 0.02,  0.09,  0.09, ...,  0.02,  0.02,  0.02],
         [ 0.02,  0.09,  0.09, ...,  0.02,  0.02,  0.02]])
  </code></pre>
  
  <p>I would like to add the array to the DataSet. My ultimate goal is to do spatial interpolation using <code>x</code> and <code>y</code> to extract interpolated values of <code>nmap</code> on a new grid. </p>