Sample code to use an unsorted map containing BDDNode (representing the
unicity table actually).
Creation of the global hashmap between indexes of variables and names of
variables.
Creation of the global unordered set which contain BDDNode's.
BDDSet is a wrapper over std::unordered_set<BDDNode> which allow
use consision and dump of an unordered set.
BDDNode's are dumpable, as well as BDDSet's.
Possibility to spawn variables through BDD::ithval (...).
Error management over spawns of variables :
 * Impossible to spawn a variable with index 0 or 1 (reserved values).
 * Impossible to spawn 2 variables with the same index.
 * Impossible to spawn variables without a prior call to BDD::init.
main.cpp contains examples of use.