title: Add string column to float matrix NumPy
body: <p>I'm looking for a method to add a column of float values to a matrix of string values.</p>
  
  <pre><code>Mymatrix = 
  [["a","b"],
   ["c","d"]]
  </code></pre>
  
  <p>I need to have a matrix like this = </p>
  
  <pre><code>[["a","b",0.4],
   ["c","d",0.6]]
  </code></pre>