Use reference polymorphism for Parameters to avoid initializing them in Preprocess constructors

[AmroAlJundi]

Also regarding making the PreprocessParams params_ data member that every preprocess has a reference, there are many reasons not to do it, but the most important one is this:
A reference data member has to refer to an object that exists outside the object instance. So it doesn't contain its own object, rather a reference at an existing object.
So if I hda this class

struct C {
  int & ref_b;
  C(int &x param): ref_b(param){}
}

int main(){
  int x = 10;
  C c(x);
  x = 11; 
  cout << c.ref_b; // prints '11'
}

Plus there's this thing about reference parameters only being initializable in initiailizer list + not being able to use r-values in the parameters of the class.