Common files for GasLab-like models

I took a stab at creating GasLab modules that can be loaded and used for other GasLab-like models. The *nls files contain the common (and complex) routines and are loaded using the __include command. You can access these files in the code tab using the Includes pull down tab.

For now, I just use 3 new routines: GasLab-init-tick-delta GasLab-new-tick-delta GasLab-collision-and-collide

I have modified a couple of GasLab codes to use these: VMC-GasLab Adiabatic Piston.nlogo VMC-GasLab Gravity Box.nlogo