Make a backup copy of the old out.xml whenever Fleur is started
People tend to forget to make backups of their old out.xml files when they start new calculations. Especially when the new calculation is a band structure calculation or other special calculation important output may get lost by this. We should make a backup copy of old out.xml files to avoid user frustration. The only question is whether we should do that always or only when a new calculation is something special. I guess "always" would be the cleaner solution but it also needs more disc space. Anyone with any opinion?