title: python-xarray: Compute the absolute values of a dataset
body: <p><code>np.fabs</code> works fine on <code>xr.DataArray</code>'s but not <code>xr.Dataset</code>'s.</p>
  
  <pre><code>data = xr.DataArray(np.random.randn(2, 3), coords={'x': ['a', 'b']}, dims=('x', 'y'))
  ds = xr.Dataset({'foo': data, 'bar': ('x', [1, 2]), 'baz': np.pi})
  np.fabs(ds)
  TypeError: ufunc 'fabs' not supported for the input types, and the inputs could not be safely coerced to any supported types according to the casting rule ''safe''
  np.fabs(ds['foo'])
  &lt;xarray.DataArray 'foo' (x: 2, y: 3)&gt;
  array([[ 0.384305,  0.161676,  0.07573 ],
         [ 0.789885,  1.299188,  1.965528]])
  Coordinates:
    * x        (x) &lt;U1 'a' 'b'
  Dimensions without coordinates: y
  </code></pre>
  
  <p>Any ideas how to apply it to a <code>xr.Dataset</code>?</p>
  
  <p>I could just loop over the variables in the <code>xr.Dataset</code> (see below) but i'm not sure if there is something more efficient</p>
  
  <pre><code>for i, var in enumerate(ds.data_vars):
      ds[var] = np.fabs(ds[var])
  </code></pre>