polytope contractor memory footprint

[soonhokong]

• reported by @pzuliani

  This is consuming 76GB and has been running for ~8 minutes
  dReal --slack-level 1 --polytope /usr/local/src/dreal3/src/tests/nra/nikos_04.smt2
  
  These two consume 94GB each, ~8 min
  dReal --slack-level 2 --polytope /usr/local/src/dreal3/src/tests/nra/nikos_03.smt2
  dReal --slack-level 2 --polytope /usr/local/src/dreal3/src/tests/nra/nikos_04.smt2
  

• possibly related to --slack-level option