title: Return a boolean DataFrame
body: <p>I would like to create a DataFrame with booleans where np.nan == False and any positive real value == True.</p>
  
  <pre><code>import numpy as np
  import pandas as pd
  DF = pd.DataFrame({'a':[1,2,3,4,np.nan],'b':[np.nan,np.nan,np.nan,5,np.nan]})
  
  DF.apply(bool) # Does not work
  DF.where(DF.isnull() == False) # Does not work
  DF[DF.isnull() == False] # Does not work
  </code></pre>