* Tests creates the root node of the PDG
* Data dependency ignores nonexecutable edges
* Non-executable edges are displayed as dashed
* Small bug in the implementation of `CFGBuilder`

Also optimized imports

* Adds API `asXArc` and `isXArc` to Arc.
* Adds Sliceable interface for graphs.
* Removed old DOT methods (toGraphvizRepresentation)
* Removed representation-related methods from graphs.
* Removed unnecessary methods.

Renamed graphs to remove redundant `Graph` at end of class names.

API changed from returning a graph to returning a set of `GraphNode#id`s.

Includes documentation for the updated classes.