title: Differences between sklearn's SimpleImputer and Imputer
body: <p>In python's <code>sklearn</code> library there exist two classes, which are doing approximately the same things:
  <code>sklearn.preprocessing.Imputer</code> and <code>sklearn.impute.SimpleImputer</code></p>
  
  <p>The only difference that I found is a <code>"constant"</code> <code>strategy</code> type in <code>SimpeImputer</code>. 
  Is there any other differences? When should be <code>Imputer</code> used instead of <code>SimpleImputer</code>?</p>