title: drop all rows when all cells are either zero or nan
body: <p>I need to remove all rows from a dataframe when all cells in each column for that row are either 0 or 'nan' (other than ID column which will be some unique value)
  I have tried the command below but no luck.
  The expected output is also pasted below.
  Thank you in advance!</p>
  <p>data</p>
  <pre><code>df = pd.DataFrame({'id':[1,2,3,4],'a':[0,0,0,1], 'b':[0,1,'nan',1], 'c':['nan',1,0,1]})
  
      id  a   b   c
  0   1   0   0   nan
  1   2   0   1   1
  2   3   0   nan 0
  3   4   1   1   1
  </code></pre>
  <p>command</p>
  <pre><code>df.loc[df[(df != 0) &amp; (df != ('nan'))].all(axis=1)]
  </code></pre>
  <p>expected output</p>
  <pre><code>    id  a   b   c
  0   2   0   1   1
  1   4   1   1   1
  </code></pre>