Fix use after free in cell attack

[vincent-noel]

Update

I updated this PR to only include cell attack, and updated the title accordingly.
Initially, when deleting a cell, I would look for all cells to see if it was a target of another cell, and if so clean the pAttackTarget.
I now modified the PR for cells to have a list of cells they are attacked by. That way, when deleting the cell, we know which cells we have to cleanup.
I think that together with #409 and #410, we have a good fix for the issues that this PR was initially tackling.

initial PR

I had some crash that I wanted to fix for a while, and finally found the issues:

• pAttackTarget is not cleaned when the target is deleted, resulting in a use after free
• neighbors are not always symmetrical, but when we delete a cell we suppose they are, resulting in a use after free
• spring_attachments are not always symmetrical, but when we delete a cell we suppose they are, resulting in a use after free.

Here the fixes I'm proposing for now:

• When deleting a cell, I check that no other cell has it as an attack target and remove it if so
• When deleting a cell, after removing it from its known neighbors, check every cell to remove from unknown (non-symmetrical) neighbors
• When deleting a cell, after removing it from its known spring_attachments, check every cell to remove from unknown (non-symmetrical) spring_attachments

Clearly, this can be improved, but it fixes the present issues.