Joint velocity limits

[Diego Ferigo]

We should take some action with gazebo-forks/dart#19 being merged. I'm opening this issue for the following reasons:

1. Reminder to introduce the support of handling velocity limits (done in feature/velocity_limit but it builds on another branch that I want to merge first).
2. Notify downstream users that their simulations could change if their models have velocity limits. In fact, before gazebo-forks/dart#19, they were not enforced by the physics engine.

In the past, when joint effort limits were enforced, we encountered some problems. Some of the results we've got were no longer valid. I'd like to avoid repeating a similar situation to happen this time.

cc @traversaro @paolo-viceconte @raffaello-camoriano @CarlottaSartore @FirefoxMetzger @AndrejOrsula

[Diego Ferigo]

Closed via #356.