Server/Physics: Implement 2d mphys system

[assofohdz]

Situation: As it is now, the physics system is 3d oriented. For Subspace Infinity, only 2D is needed

1. Extend PhysicsSpace w. constructor taking a new TandemContactResolver
2. Overwrite PhysicsSpace::resolveContacts( double step ) and use new resolver
3. Overwrie TandemContactResolver::resolvePenetration and TandemContactResolver::resolveVelocity and copy-paste, and then cancel out any y-calculations

or

1. Extend RigidBody and overwrite Integrate() and cancel the y-movement there