title: Summarise dataframe to include all unique values in a grouping
body: <p>I'd like to summarise a dataframe such that a column contains a string of the unique values within a particular group. So using the iris dataset:</p>
  
  <pre><code>iris %&gt;%
    group_by(Species) %&gt;%
    summarise(mPW=mean(Petal.Width))
  </code></pre>
  
  <p>This gives the mean of Petal.Width grouped by Species. But what if I want as an output all the values that were used to calculate that mean. I want those unique values in a list though not in the R meaning of a list. I tried this but obviously that was wrong:</p>
  
  <pre><code> iris %&gt;%
     group_by(Species) %&gt;%
     summarise(lPW=paste(Petal.Width, sep=","))
  </code></pre>
  
  <p>Here is a truncated desired example dataframe output. Note the desired output for <code>LPW</code> is a character object:</p>
  
  <pre><code> Species lPW
   setosa  0.1,0.2,0.3,0.4,0.5,0.6
   ....
  </code></pre>
  
  <p>I'm not set on <code>dplyr</code> solution. This is just the way I normally work. </p>
  
  <p>Thanks in advance.</p>