Refactoring getSourceVersion() -> getSourceVersionNumber() (and same for target) inf MavenLabeledEdge
Suppression of getDependencies() and getUsages() in MavenMultigraph (and impl); adding of edgesOf() and getEdgeSource/Target() in MavenMultigraph (and impl)
Implementation and testing of computeDependencies(), computeAllDependencies(), computeUsages() and computeAllUsages() in MavenMetrics
Tests cleaning (need more testing) in MavenMultigraphLabeledTest