Allow separate configuration of tasks and constraints

[dmronga]

Currently, what is known as a "task" in WBC literature is entitled "constraint" in the software. This is a legacy from the iTaSC software, where this naming scheme has been used. In fact, using a QP solver, tasks are usually included in the cost function, so this naming is actually wrong. Also, "constraints" of a QP solver are currently hard coded in the WBC scene, i.e., they are not configurable.

• Rename "Constraint" to "Task" (this should not effect .yml config files in Rock)
• Add configuration option for hard constraints, e.g., joint velocity. This is specific to the type of WBC used. Constraints should be generic as tasks, so that we can also constraint sth. like self collisions or CoM position