Use Arc<ScipPtr> in place of all scip pointer uses

[mmghannam]

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[mmghannam]

There's a memory leak that I traced back to the tests that use model.clone_for_plugins() I can't seem to figure it out. I think the way the callbacks are implemented I'm not decrementing dropping the Arc correctly. @Andful any idea what could be wrong here?

[Andful]

Yeah, looking at it seemed like there is a memory leak. From what I saw, you double Box the trait. I think once is enough. But I have to look into it deeper. I would make an issue for now, and in the future, make a different pull request for it.

[mmghannam]

Yeah, looking at it seemed like there is a memory leak. From what I saw, you double Box the trait. I think once is enough. But I have to look into it deeper. I would make an issue for now, and in the future, make a different pull request for it.

I think I did that because I had compilation issues without this indirection. But maybe I did something wrong back then. Anyway, I'm hesistant to merge this with this memory leak but it's limited to the use of callbacks which is not the most common use case and it will enable much safer use of the library. So let's merge it and deal with the memory leaks later as you suggested.

Update: I added issue #159 to track this memory leak.