Redefine damage rate

Define damage rate as damage/attack. See MathCancer#223.

core/PhysiCell_cell.cpp

@@ -1404,7 +1404,7 @@ void Cell::attack_cell( Cell* pCell_to_attack , double dt )
  { 
  // std::cout << this->type_name << " attacks " << pCell_to_attack->type_name << std::endl;
  // 
- pCell_to_attack->state.damage += phenotype.cell_interactions.damage_rate * dt; 
+ pCell_to_attack->state.damage += phenotype.cell_interactions.damage_rate; 
  pCell_to_attack->state.total_attack_time += dt; 
  }
  return;