Remove direct output to file from GUI.
Move parts of Particle2DPanel to new class SimulationAnimation.

Particle2DPanel only deals with the graphical output.

SimulationAnimation deals with the animation logics.
Create new class PhaseSpacePanel.

This panel shows graphically the one-dimensional phase space (x vs. vx).
Fix bug "array index out of bounds"

Index stays within grid size.

Patch by Clemens Müllner.
Merge branch 'upstream' into graphicspanel

Fixes a critical bug.