title: Replace xarray coordinates with another coordinate
body: <p>I try to replace two coordinates with the same length in a xarray.Dataset by using one coordinate for both of them. I do not care about the old coordinates or its values; I simply want to replace them. Is there any convenient method for this?</p>
  
  <pre><code>import xarray as xr
  ds = xr.Dataset({
      "a": ("a_coord", [0, 1, 2]),
      "b": ("b_coord", [50, 10, 20]),
      "a_coord": [4, 8, 9],
      "b_coord": [3, 1, 4],
      "new_coord": [0, 1, 2],
  })
  # Get rid of a_coord and b_coord and replace both with new_coord, so a and b
  # have new_coord as coordinate.
  # But how?
  </code></pre>
  
  <p>I tried <code>rename</code>, but it returns a conflict error (since new_coord already exists). </p>
  
  <pre><code>ds.rename({
      "a_coord": "new_coord",
      "b_coord": "new_coord",
  })
  </code></pre>
  
  <p>Do I have to assign the new coordinate explicitly to each data variable that was depended on one of the old coordinates? Since I have many data variables (not just a and b) and other coordinates, this could become messy.</p>