[las] design and use strategy files
Imported issue: Initially reported by @zimmerma in https://gforge.inria.fr/tracker/?group_id=2065&aid=15017
for the cofactorization, we should design some strategy files, and use them in las.
We could reuse the format used for RSA768: on each line we have the size in bits of both cofactors, and the corresponding factoring methods used.