title: Via/ViaMat/to/toMat in Akka Stream
body: <p>Can someone explain clearly what are the difference between those 4 methods ? When is it more appropriate to use each one ? Also generally speaking what is the name of this Group of method? Are there more method that does the same job ? A link to the scaladoc could also help. </p>
  
  <p>-D-</p>