title: Add column of dense vectors over a groupby in pyspark 2.2 or 2.3
body: <p>I am using Pyspark 2.2.</p>
  <p>I have a input table like this:</p>
  <pre><code>tag | features
  1   | [1,0,0,2]
  2   | [1.5,0,1,0]
  2   | [0,0,1,0]
  </code></pre>
  <p>Need output like this</p>
  <pre><code>tag | sum(features)
  1   | [1,0,0,2]
  2   | [1.5,0,2,0]
  </code></pre>
  <p>Element wise addition needs to happen.
  So far I have is:</p>
  <pre><code>df.groupBy('tag').agg(F.sum('features')).show(5,0)
  </code></pre>
  <p>But this gives me an error:</p>
  <pre><code>cannot resolve 'sum(`features`)' due to data type mismatch: function sum requires numeric types, not ArrayType(FloatType,true)
  </code></pre>
  <p>Any help would be appreciated.</p>