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
Remove .travis.yml
Add .travis.yml.
Lower Cmake minimal version requirement for Travis.
Add boost to Travis configuration file.
Don't try to use travis-ci.

The provided environment lacks recent version of gcc and boost.
Export an SDD's statistics to a JSON file.
Fix unsigned int/std::size_t mismatch in some tests.
Keep the peak number of unified elements.
Extract unique_table::statistics, a manager is no longer printable.
Statistics are internal stuff.
Export a JSON file for the statistics of a manager.
Handle cache statistics for the JSON export of a manager.
Merge branch 'master' into force
Remove inclusion of unused header.
Add the possibility to interrupt an evaluation.