title: Delete columns where all values are 0
body: <p>I have a numeric matrix with <code>15000</code> columns. I want to completely remove the columns where <em>all</em> values are <code>0</code>.</p>
  <pre><code>     col1     col2     col3     col4
  row1  1        0        0        1
  row2  3.4      0        0        2.4
  row3  0.56     0        0        0
  row4  0        0        0        0
   
  </code></pre>
  <p>Here I want to delete columns <code>col2</code> and <code>col3</code>, and keep the rest.
  How can I do it with R?
  Thanks</p>