Sort the configurations in place. items with lowest memory per proc come first.

def sort_by_mem_per_proc(self, reverse=False):
        """Sort the configurations in place. items with lowest memory per proc come first."""
        # Avoid sorting if mem_per_cpu is not available.
        if any(c.mem_per_proc > 0.0 for c in self):
            self._confs.sort(key=lambda c: c.mem_per_proc, reverse=reverse)
        return self