Implement propagation

[celeritas-project]

Implement propagation

[sethrj]

From today's discussion of Propagator interface:

• Both linear and magnetic field propagators will have two operator(), one with a maximum step distance, one without
• Result of both operator() will be a struct returning the actual distance traveled, and (if it crossed into a new volume) the new updated volume ID
• (Maybe?) If it moves outside, return an unassigned volume ID VolumeId{}

struct result_type
{
    real_type distance; //!< Distance traveled to min(input limit, boundary)
    VolumeId volume; //!< Post-propagation volume
};

// Input for propagation kernel: maximum distance to next physics interaction
// (or in the raytrace/rasterizing case, distance to next pixel)
result = propagate(limit) -> (distance actually traveled, VolumeId)
propagate(nolimit) -> (same)

// hit a new geometry volume *before* we hit limit:
result = {distance_traveled, geo.volume_id()};
// hit limit before hitting geometry boundary
result = {limit, VolumeId{}};
// no-limit case:
result = {distance_traveled, geo.inside() ? geo.volume_id() : VolumeId{}};

Hypothetical usage with an amazing goto:

__global__ void move_to_new_material(...)
{
    GeoTrackState geo(...);
    Propagator propagate(geo);
    MaterialTrackState mat(...);

    real_type physics_distance_limit = physics[thread_id];
    real_type accum_distance = 0;
    MaterialId matid = mat.id();

prop:
    auto result = propagate(physics_distance_limit);
    accum_distance += result.distance;
    if (result.volume)
    {
        // Moved to a new cell; check the new material
        if (volume_to_mat[result.volume.get()] == matid)
        {
            // Material didn't change; keep propagating
            physics_distance_limit -= result.distance;
            goto prop;
        }
    }

    return result.distance_traveled;
}