ENH : Use the AddProcessToWatch event to store the process of which we want to report the progress

          - a AddProcessToWatch is instanciated, setting the process and the description
          - the event is invoked
          - the class that are responsible of reporting the progress must observe this event
          - a callback is executed when the event observed is invoked, and then we can get the
          - pointer to the processObject and the description

          Using event allow us to use the right pointer to he ProcessObject we want to observe, avoiding
          observing null pointers, pointers relative to a previous execution, and to avoid bad RequestedRegion
          propagated in the pipeline errors.