title: add a dask.array column to a dask.dataframe
body: <p>I have a dask dataframe and a dask array with the same number of rows in the same logical order. The dataframe rows are indexed by strings.
  I am trying to add one of the array columns to the dataframe.
  I have tried several ways all of which failed in their particular way.</p>
  <pre><code>df['col'] = da.col
  # TypeError: Column assignment doesn't support type Array
  
  df['col'] = da.to_frame(columns='col')
  # TypeError: '&lt;' not supported between instances of 'str' and 'int'
  
  df['col'] = da.to_frame(columns=['col']).set_index(df.col).col
  # TypeError: '&lt;' not supported between instances of 'str' and 'int'
  
  df = df.reset_index()
  df['col'] = da.to_frame(columns='col')
  # ValueError: Not all divisions are known, can't align partitions. Please use `set_index` to set the index.
  </code></pre>
  <p>and a few other variants.</p>
  <p>What is the right way to add a dask array column to a dask dataframe when the structures are logically compatible?</p>