title: Column-wise string concatenation of multiple rows in pandas.DataFrame with specific separator
body: <p>is there an <strong>efficient</strong> way to concatenate strings column-wise of multiple rows of a DataFrame such that the result is a single row whose value for each column is the concatenation of each value of the same column of all given rows?</p>
  <p><strong>Example</strong></p>
  <p>Combine the first four rows as explained above.</p>
  <pre><code>&gt;&gt;&gt; df = pd.DataFrame([[&quot;this&quot;, &quot;this&quot;], [&quot;is&quot;, &quot;is&quot;], [&quot;a&quot;, &quot;a&quot;], [&quot;test&quot;, &quot;test&quot;], [&quot;ignore&quot;, &quot;ignore&quot;]])
  &gt;&gt;&gt; df
          0       1
  0    this    this
  1      is      is
  2       a       a
  3    test    test
  4  ignore  ignore
  </code></pre>
  <p>Both accepted results:</p>
  <pre><code>          0              1
  0  this is a test  this is a test
  </code></pre>
  <pre><code>          0
  1  this is a test
  2  this is a test
  </code></pre>