Optimize dependency graph layout

[AdrienDebrie]

Currently, a first optimization is implemented: it removes empty actions, and it also remove useless edges in the graph such as

A->B->C
and
A->C,

This last edge is useless in the dependency graph and can therefore be removed.

However at least one other optimization can be made:

The following graph :

A1 -> Z1
A1 -> Z2
...
A1 -> Zp
...
A2 -> Z1
A2 -> Z2
...
A2 -> Zp
...
An -> Z1
An -> Z2
...
An -> Zp

requires n*p edges. It is equivalent semantically to this one:

A1 -> B
A2 -> B
...
An -> B

B -> Z1
B -> Z2
...
B -> Zp

which only requires n+p edges.

Note : graph optimization should be an option (on by default). It changes the graph, and therefore may make dependency reading more difficult.