Added collision handler for Huayno OK algorithm

[ErwanH29]

Integrated the collision detection algorithm from the various SHARED_COLLISION Huayno integrators into the OK integrator.
Note: This collision checker does not detect collisions between two massless particles.

[LourensVeen]

It seems like this is effectively the contents of the detect_collisions() function from evolve_shared_collisions.c, copy-pasted into evolve_ok.c. This is unfortunately done quite frequently, but it's not good programming practice.

The point of functions is that they can be called from multiple places in the code, thus avoiding duplication. And that is important, because duplicates will diverge over time. Imagine that someone finds a bug in the original function and submits a fix. Will they know that there's a copy somewhere that should also be fixed?

Perhaps you can modify this to call that function, rather than copy-paste it? If you need to change the function, you'll have to carefully check where else it's called and make sure that those cases will still work as well, of course.

[ErwanH29]

I made a new file which hosts the collision detection function and with which both evolve_shared_collisions.c and evolve_ok.c call. The algorithm neglects collisions between two massless particles to save computing time at the detriment that perhaps this isn't an assumption that some users would want. If that is a bother I can rewrite the code to include it.

I hope the changes are fine. I tested collisions using either integrators and I don't see a change in performance.

[rieder]

This PR needs at least a test to show that this collision detection actually works before it can be merged.

[stale]

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