Correcting the prepare, complete and the main method, changing the order of calculating the position, velocity and acceleration and introducing new variables that are needed for storing the forces for the complete method
Introducing new methods that are used in the LeapFrog algorithm
Correcting errors in the "write" method that were created from before with the diving of some classes
Correcting a comment
Merge pull request #43 from okapetan/fixdamped

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.