Extend option to add variables as parameters or optimized to the TaskModel

[joaomoura24]

In the RobotModel we are adding the ability to provide a list of param_joints, so that we can optimize a subset of the robot joint positions and/or velocities.
This concept could be generalized and extended to the the Model, extending it to the TaskModel.

Because we are currently not making use of the TaskModel for specific problems, it is not so useful now to extend this feature, but once we have an example of usage of the TaskModel, we should work on this.