finishing branch ropt_dev
Imported issue: Initially reported by Shi Bai in https://gforge.inria.fr/tracker/?group_id=2065&aid=21540
Ropt uses different levels of approximations in the algorithm. In the first stage, the tree-based method is used to generate 'crude' good sublattices; in the second stage, the root sieve is done for each sublattice found from the first stage. Reducing the number of sublattices in the first stage (while not losing the good poynomials) leads to a faster root sieve implementation. The branch ropt_dev is onto that; with also a pthreading implementation. This is logged here as a to-be-finished project.