Mega commit: object trees and their connections.

* VariableAction stores pending connections between trees.
* Update JavaParser from 3.17.0 to 3.19.0
* Object trees in assignments and return statements are linked to the appropriate expressions.
* Return trees are linked interprocedurally.
* ReturnStmt variable actions are now all generated by VariableVisitor.
* CallNode.Return are now generated by VariableVisitor instead of the SDG after the interprocedural finders.
* Actual-in nodes are now generated based on the expressions used, and then linked to the actual-in generated by the InterproceduralUsageFinder.
* We now accept calls as scope or argument of another call.
* ClassGraph is now a singleton. ClassGraph#newInstance() generates a new one.
* ClassGraph can generate a complete ObjectTree for a given type.
* MemberNode's parent can now be any node, and can be set after creation.
* New slicing algorithm for the JSysDG.
* Added interprocedural object-flow arcs.
* Added ObjectFlow and Flow dependency arcs.
* Introduced test to guarantee that interprocedural arcs are either input xor output.
* Updated tests