title: xarray select nearest lat/lon with multi-dimension coordinates
body: <p>I have a xarray dataset with irregular spaced latitude and longitudes coordinates. My goal is to find the value of a variable at the point nearest a certain lat/lon.</p>
  
  <p>Since the <code>x</code> and <code>y</code> dimensions are not the lat/lon values, it doesn't seem that the <code>ds.sel()</code> method can be used by itself in this case. Is there a xarray-centric method to locate the point nearest a desired lat/lon by referencing the multi-dimensional lat/lon dimensions? For example, I want to pluck out the SPEED value nearest <code>lat=21.2</code> and <code>lon=-122.68</code>.</p>
  
  <p>Below is an example dataset...</p>
  
  <pre><code>lats = np.array([[21.138  , 21.14499, 21.15197, 21.15894, 21.16591],
                   [21.16287, 21.16986, 21.17684, 21.18382, 21.19079],
                   [21.18775, 21.19474, 21.20172, 21.2087 , 21.21568],
                   [21.21262, 21.21962, 21.22661, 21.23359, 21.24056],
                   [21.2375 , 21.2445 , 21.25149, 21.25848, 21.26545]])  
  
  lons = np.array([[-122.72   , -122.69333, -122.66666, -122.63999, -122.61331],
                   [-122.7275 , -122.70082, -122.67415, -122.64746, -122.62078],
                   [-122.735  , -122.70832, -122.68163, -122.65494, -122.62825],
                   [-122.7425 , -122.71582, -122.68912, -122.66243, -122.63573],
                   [-122.75001, -122.72332, -122.69662, -122.66992, -122.64321]])
  
  speed = np.array([[10.934007, 10.941321, 10.991583, 11.063932, 11.159435],
                    [10.98778 , 10.975482, 10.990983, 11.042522, 11.131154],
                    [11.013505, 11.001573, 10.997754, 11.03566 , 11.123781],
                    [11.011163, 11.000227, 11.010223, 11.049   , 11.1449  ],
                    [11.015698, 11.026604, 11.030653, 11.076904, 11.201464]])
  
  ds = xarray.Dataset({'SPEED':(('x', 'y'),speed)},
                      coords = {'latitude': (('x', 'y'), lats),
                                'longitude': (('x', 'y'), lons)},
                      attrs={'variable':'Wind Speed'})
  </code></pre>
  
  <p>The value of <code>ds</code>:</p>
  
  <pre><code>&lt;xarray.Dataset&gt;
  Dimensions:    (x: 5, y: 5)
  Coordinates:
      latitude   (x, y) float64 21.14 21.14 21.15 21.16 ... 21.25 21.26 21.27
      longitude  (x, y) float64 -122.7 -122.7 -122.7 ... -122.7 -122.7 -122.6
  Dimensions without coordinates: x, y
  Data variables:
  SPEED      (x, y) float64 10.93 10.94 10.99 11.06 ... 11.03 11.03 11.08 11.2
  Attributes:
      variable:  Wind Speed
  </code></pre>
  
  <p>Again, <code>ds.sel(latitude=21.2, longitude=-122.68)</code> doesn't work because latitude and longitude are not the dataset dimensions.</p>