WIP: Added Naive summary arcs builder

[jacosro]

Added

• SummaryArc
• Missing isXArc and asXArc methods in Arc class
• tfm.graphs.sdg.sumarcs package, which contains:
  • SummaryArcBuilder, an abstract class
  • NaiveSummaryArcBuilder, naive summary arcs builder which extends from SummaryArcBuilder

Details on the naive summary arcs builder

Computes the summary arcs by checking the reachability from formal_out to formal_in parameters, and then for each actual_out finds the corresponding actual_ins and adds the SummaryArc to the graph

Partially solves #7