title: replace commas to decimal points in DataFrame columns to make them numeric
body: <pre><code>import pandas as pd    
  df = pd.DataFrame([["one", 1.2, "", "4,3"],
     ["two","1,7", "2,4", 0.55],
     ["three","", 5.4, "3,9"]], 
     columns=['a','b','c','d'])
  </code></pre>
  
  <p>How can I replace the commas in the columns b to d to points and make these columns numeric?
  Column a (or any number of leading columns) should be retained untouched (can contain commas as well)</p>