title: keep only lowest value per row in a Python Pandas dataset
body: <p>In a Pandas dataset I only want to keep the lowest value per line. All other values should be deleted.
  I need the original dataset intact. Just remove all values (replace by NaN) which are not the minimum.</p>
  
  <p>What is the best way to do this - speed/performance wise.</p>
  
  <p>I can also transpose the dataset if the operation is easier per column.</p>
  
  <p>Thanks
  Robert</p>