Remove duplicate call.
Fix bug for initial conditions of pair:
The problem was in MainControlApplet.setSlidersValue():
after initialization of Simulation, it called particlePanel.s.grid.changeSize(...) which reset the grid.

Because the ConstantForce was not linked, actually setSlidersValue() crashed when
force.ex was called, so that changeSize(...) was not called in that case.

After removing changeSize(...), it correctly works also with additional ConstantForce.
Fix bug: in MainControlApplet.setSlidersValue() set slider value
according to current setting in simulation.

Previously the time step was hardcoded to 0.5.