Creation of the C++ generation task for PiMM models.
Added the CPPCodeGenerationVisitor Scala class skeleton.

This class is responsible for visiting a PiMM model and generate C++ code for it, using a Scala StringBuilder and pattern matching.
Added generation of subgraphs (hierarchical actors).
Added generation for classical actors (Actor and AbstractActor) and for data ports (DataInputPort and DataOutputPort).
Added generation for FIFOs (Fifo).
Added comments on the use of PortDescriptions in visitFifo.
Added generation for parameters.
Added a generic trait for PiMM models visitors.

Should be moved to a more generic project (org.ietr.preesm.experiment.pimm.visitor.scala) if reused in other projects.
Added licence and copyrights informations to:
-Activator.java;
-PiMMCppGenerationTask.java;
-CPPCodeGenerationVisitor.scala;
-PiMMVisitor.scala.
Added DataInputInterfaces and DataOuputInterfaces generation.
Added generation for InterfaceActors.
Added pre-processing for Dependencies and Fifos generation.

Preprocessing is done by Scala class CPPCodeGenerationPreProcessVisitor.
Added license and copyrights informations in CPPCodeGenerationPreProcess.scala.
Implemented methods of class PiMMCppGenerationTask.
Corrected a bug in visitFifo (we did not check the existence of a Delay before accessing its Expression).
+ refactorings (Added a scala class responsible for name generation of the different C++ elements).
Added scala class CPPCodeGenerationLauncher which generate the whole content of the C++ file, from includes to methods.
Registered the C++ generation as a workflow task.
Corrected a bug: trying to get the top element of an empty stack.
Corrected few missing new lines in generated code.
Refactorings: removed the PiGraph parameter of CPPCodeGenerationVisitor constructor, renamed some methods.