collision checkbox on by default
Merge pull request #1 from Aerlinger/anthony

Merged Anthony's fork adding a symplectic Euler integrator, radial gravitation and an improved collision response
refactored integration methods. changes relationship between mass and particle radius
added output option and changed a vector field name
removed restitution input (defined in code). added integration method choice. extracted collisions detection and drawing. collision detection can now be iterated.