GroupBy after the traverse

[jejemontagne]

Add a feature allowing to group the result after chained "traverses" according to a given function like the following example:

The GroupBy takes two parameters :

• a function f : KObject -> long describing the filter t o apply to group the elements given by the chained traverse
• a KCallback : function describing the action to do after the traverses and groupBy has been done

Obj.traversal().travserse(REF_SUBOBJ1).travserse(REF_SUBOBJ2).groupBy( element -> {return element.uuid();} , KCallback(){ DO });}