Buckingham Potential Added

Buckingham potential has been added. Switching between different
potential has to be done manually inside the code.
Also, the Lennard-Jones potential is pre-calculated and looked up from
an array when needed.
Switch Lennard-Jones and Buckingham via GUI

Now, we can switch between Lennard Jones potential and Buckingham
potential via the drop down menu in the GUI.
Moved main sim cod from updatefixed() to update() for huge performance improvement