tweaked main.cpp for our usage:

- added Point and Spring structs
- added method InitPoints() to initialize spheres (points) in a grid
- added method InitSprings() to initialize springs between points
- added method DrawMassSpringSystem() to draw the initialized points and springs
- added additional variables to TweakBar like mass, springStiffness and damping

ToDo:
- method to compute external forces
- method to compute internal forces
-> method that uses euler integration
-> method that uses midpoint integration
- method to update position of points and springs
- be happy if it works