Pipelines and algorithms

[cbonesana]

Long time ago, for the Belief Propagation we added the Algorithm Interface.
With my last commit of today, with the help of @davidhuber , I also added a pipeline class called Pipe. The idea is to use the Pipe to build the Belief Propagation algorithm as a Pipeline that can produce a self-updatable JunctionTree.

Doing so I thought that we need some kind of interfaces as in sklearn or Spark:

• a Transformer interface that act like the current Algorithm interface and transform one input to an output;
• an Estimator interface that take some inputs but does not produce any kind of output.

(Names are from Spark)

Will this be somewhat helpful?