Correcting the LeapFrog damped algorithm

The LeapFrog damped algorithm was corrected: 
-the order of calculating the position, velocity and accceleration was changed;
-the prepare and complete methods were also corrected and the help coefficients in these methods.

New methods in the Particle class were introduced, that are needed in preprare and complete methods from the LeapFrog damped class.

A couple of errors in Particle2DPanel in the write method were corrected, that were done from before with the dividing of some classes in the physics library.