Reduce the SWIFT memory usage during a VR call by freeing the foreign particle buffers

We additionally zero all the particle pointers in the foreign cells such that any future attempt at using them (by mistake) crashes cleanly.

This might help with @jch's crash on Curie.

Mmmh this just crashed on a hydro run so I need to look into it a bit more...

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• 9d9f8e8e - Cancel the pointers in all the cells for which we have proxies and not just in…

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That's all good now. The previous version was a bit too lazy with its pointer resetting strategy.

@jch have you used this? Did it help?

I tried this and it did reduce peak memory use substantially (from something like 90GB down to 75GB). It didn't prevent the crash that I had in velociraptor but I now think the crash was probably not due to lack of memory.

Thanks, I'll accept this.

merged

mentioned in commit 728c65e8