fix(realpaver/rp_propagator): wrap proof output with verbose check
fix(dsolvers/icp_solver): missing build_explanation()
fix(dsolvers/icp_solver): print polarity in build_explanation
feat(realpaver): implement better explanation

 - rp_sat_hull_* always marks a constraint as used
 - rp_ctr_num*_unfeasible marks a constraint as used if it is unfeasible
   with a current box. Otherwise, do not change its marker.
 - if rp_newton_factory is used, it marks all constraints that it uses
   as used.