title: Only remove entirely empty rows in pandas
body: <p>If I have this data frame:</p>
  <pre><code>d = {'col1': [1, np.nan, np.nan], 'col2': [1, np.nan, 1]}
  df = pd.DataFrame(data=d)
  
      col1    col2
  0   1.0     1.0
  1   NaN     NaN
  2   NaN     1.0
  </code></pre>
  <p>and want to drop only rows that are empty to produce the following:</p>
  <pre><code>d = {'col1': [1, np.nan], 'col2': [1, 1]}
  df = pd.DataFrame(data=d)
  
      col1    col2
  0   1.0     1
  1   NaN     1
  </code></pre>
  <p>What is the best way to do this?</p>