title: pandas groupby count the number of zeros in a column
body: <p>I have a dataframe, e.g:</p>
  <pre><code>Date             B           C   
  20.07.2018      10           8
  20.07.2018       1           0
  21.07.2018       0           1
  21.07.2018       1           0
  </code></pre>
  <p>How can I count the zero and non-zero values for each column for each date?
  Using <code>.sum()</code> doesn't help me because it will sum the non-zero values.</p>
  <p>e.g: expected output for the zero values:</p>
  <pre><code>Date          B         C
  20.07.2018    0         1
  21.07.2018    1         1
  </code></pre>