.xyz file is appended to after each simulation

[mec07]

We want to append to the .xyz file so that we can track a particle. The problem is that the file is not wiped clean at the beginning of each simulation. This should be easy to fix.

[mb4512]

In PLATO this is not a bug, but a feature. We need to decide whether we want to keep this.

Your call. Whatever makes most sense for your project