Build the hypergraph necessary to use the FORCE ordering method.
Implemented FORCE ordering strategy.

Need to be validated now.
Export DOT of hypergraph or debugging purposes.
Tuning experiment: push variables in a reverse order.
Merge branch 'master' into force
Merge branch 'master' into force
Merge branch 'master' into force
Merge branch 'master' into force
Merge branch 'master' into force
Extract FORCE vertex and hyperedge into their own files.
Provide the possibility to directly give the hypergraph to FORCE.

The code is ugly, a lot of copy-paste. It needs a full cleanup.
Merge branch 'master' into force
Merge branch 'master' into force
Merge branch 'master' into force
Make hyperedge and vertex classes.
Remove FORCE strategy on homomorphism.

Will come back on this later.
Hide the construction of the FORCE's hypergraph.