Graph data structure optimized to model execution

Nodes contains 4 pointers for linking:

* prev/next : horizontal link
* from/to: vertical link

Link are annotated with LinkType, that represents the state of the task or the
resource involved in the execution.

This data structure is better than general graph:

* HashMap lookup for the node list related to an object
* Nodes related to the same object are kept in an ArrayList, sorted by
  timestamps, allowing efficient binary search
* Direct access to neighbor

It allows to create direct links between two state changes.
update target
Update ignore directory results
Add dot output for zgraph
Support partial graph output
Skel for test graph factory
Convert link fields to array

There are 4 references, put them in array for easy looping and direction
testing.
temp
Add test case and critical path skeleton
Re-add cloner lib

We always need cloner at some point
Add skel of basic graph operations
Add support for iter of graph
Fix iter test
Add union operation
Partial right left union
Add match checking
Add insert before/after operations
Add merge operation
Add align operation
Do not implement minimize now