Correct the amount of RAM used by 2P preprocessing

2p-preprocessing/preprocessing.cpp

@@ -431,7 +431,7 @@ int main(int argc, char *argv[])
 
 	const size_t db_nitems = 1ULL << expo;
  size_t RAM_needed = 0;
- RAM_needed = n_threads * 18 * db_nitems;
+ RAM_needed = n_threads * 35 * db_nitems;
  std::cout << "RAM needed = " << RAM_needed << " bytes = " << RAM_needed/1073741824 << " GiB" << std::endl;
  size_t n_batches = std::ceil(double(RAM_needed)/(1073741824 * maxRAM));
  std::cout << "n_batches = " << n_batches << std::endl;