External forces for rigid body dynamics

Currently the rigidBodyDynamics library only supports springs, dampers and a prescribed rotation. I'd love to see a functionality that allows for an external force restraint. E.g for simulating a rigid body that is being pushed by a certain force. Which might be especially useful for overset simulations. Attached is this restraint, which I'd like to see added to src/rigidBodyDynamics/restraints. The force is time and direction variable via function1. externalForce.zip

Best regards

Stephan

## Reattaching the author to the issue ticket: @Goeke ##