refactor

src/main/java/tfm/graphs/CFGGraph.java

src/main/java/tfm/graphs/Graph.java

src/main/java/tfm/graphs/PDGGraph.java

src/main/java/tfm/nodes/CFGNode.java

src/main/java/tfm/nodes/Node.java

src/main/java/tfm/nodes/PDGNode.java

src/main/java/tfm/variables/VariableExtractor.java

src/main/java/tfm/variables/VariableSet.java

src/main/java/tfm/variables/VariableVisitor.java

src/main/java/tfm/visitors/CFGVisitor.java

src/main/java/tfm/visitors/PDGVisitor.java