Modified the Timing class to handle expressions for timings.

Added a new attribute stringValue containing the expression of the timing.
Added a new attribute isEvaluated which is true if the expression contained by stringValue has been evaluated (all variables have been fixed and the value has been computed), false otherwise.
If isEvaluated is at true, we can use the long contained by the time attribute.
Refactoring (format + transforming some if/else if to switch)
Refactoring (removed an useless TODO)
Modified the ScenarioWriter wrt. changes in the Timing class (introduction of expressions).
Added a ParameterValueManager to PreesmScenario to handle Parameters from PiGraphs.

Added a ParameterValue class which handles one parameter (its name, type and value/expression)
Added a ParameterValueManager which stores all the ParameterValues for one scenario
Modified PreesmScenario to add it a ParameterValueManager attribute
Modified ScenarioParser and Writer wrt. changes in PreesmScenario (introduction of ParameterValues)
Renamed getAlgorithm to getSDFGraph and added a getPiGraph method in PreesmScenario in order to handle both IBSDF and PISDF algorithms.
Added to PreesmScenario a number of executions of the top (outter) graph, when simulating a PiGraph.

+Serialization of this number
Added an update to PreesmScenario, taken from PiScenario.
Added images from PiScenario UI.
Added PiParametersPage to ScenarioEditor, taken from PiScenario UI.
Corrected the createIntegerSection method in order to use it for several different attributes of a PreesmScenario (namely transfer size and number of top executions, but it can now be used for any integer attribute).
Corrected Timing to try to evaluate the stringValue every time it changes.
Added ".pi" extension to authorized extensions for algorithm files.
Added an helper method getHierarchicalActorFromPath to PiGraph, which look for an Actor through the graph hierarchy.
Format
Numerous changes to include PiGraphs in PreesmScenario:

-Changed TimingManager.getTimingOrDefault method to return a Timing object and not a long, since a Timing can no be chracterized by an expression
-Added a TimingManager.setTiming method which accept a String as value rather as a long
-Added a PiGraph attribute to ScenarioParser to handle PISDF algorithms, and modified ScenarioParser to handle this PiGraph
-Modified TimingsPage to add columns to the timings table (parsing, evaluation, variables) and change the time column to an expression one
-Modified SDFTableLabelProvider to provide text and images for Timings, based on PiScenario
Merge branch 'develop' into merging-scenarios
Added two methods to determine whether a Scenario use a PISDF algorithm or IBSDF one.
Renamed SDFTableLabelProvider to TimingsTableLabelProvider and modified it to handle PISDF elements :

-Added a method getPISDFColumnImage called by getColumnImage if the current scenario points to a PISDF algorithm
-Added a method getPISDFColumnText called by getColumnText if the current scenario points to a PISDF algorithm
-Added a method getIBSDFColumnText called by getColumnText if the current scenario points to a PISDF algorithm

Each of these methods allows to display the correct text/image depending of the algorithm we handle

Renamed SDFListContentProvider to PreesmAlgorithmListContentProvider and modified it in order to return either SDFAbstractVertex from a IBSDF algorithm, or AbstractActors from a PISDF algorithm.