particle property iterators

[RudolfWeeber]

Create iterators over single particle propreties and common groups of particle properties

auto positions = PositionIterator(cellstrucutre.local_particles());
for (std::vector3d& pos: positions) {
};

This can, to my understanding, be implemented by a boost transform iterator.

For combinations such as
PositionChargeIterator,

a design choice needs to be made:
Either one uses a boost zip iterator, then the individual properties (position, velocities) are numbered. (zip itertaor returns tuples)
Or we create proxy classes which forward the poiters and references as named members

class PositionChargeRefProxy() {
   PositionVelcoityProxy(Vector3d& pos_ref, Vector3d charge_ref) {
   m_pos_ref = pos_ref;
   m_charge_ref = charge_ref;
   };
   Vector3d& m_pos_ref;
   Vector3d& charge_ref;
   Vector3d& pos() { return pos_ref; };
   doube& charge() { return m_charge_ref; };
};

[RudolfWeeber]

Here's a sketch generated using ChatGTP. May or may not work, but looks OK

#include <boost/iterator/transform_iterator.hpp>

class Particle {
public:
  const Vector3d& pos() const {
    return position_;
  }

private:
  Vector3d position_;
};

// Define a function object to extract the pos() member from a Particle object
struct ExtractPos {
  const Vector3d& operator()(const Particle& particle) const {
    return particle.pos();
  }
};

int main() {
  std::vector<Particle> particles = /* ... */;

  // Create a transform iterator that extracts the pos() member from each Particle object
  auto pos_iterator = boost::make_transform_iterator(particles.begin(), ExtractPos{});

  // Iterate over the pos() members of the particles
  for (auto pos : pos_iterator) {
    // Do something with pos, which is a reference to a Vector3d object
  }

  return 0;
}