unused old_position

[rheiland]

• in Cell::update_position, comment out the unused std::vector<double> old_position(position) to avoid confusion and perhaps speed up a simulation

[drbergman]

Makes complete sense to remove this. I second that this is unnecessary.

I personally would prefer to delete this line and the three commented out lines below. It seems they offer an explanation for this variable: that it was created for debugging/diagnostic purposes.