Subconstraint implementation

[EnricoMingo]

I am trying to add this feature in CartesI/O based on the sub_constraint branch of OpenSoT, however, I am finding some problems understanding the flow during stack construction so I am not sure I have implemented correctly all the interfaces. In particular, I can not find where the YAML is parsed. @alaurenzi could you check the sub_constraint branch of CartesianInterface and give me some pointers?

[alaurenzi]

It should not require much code writing, as in Cartesio a constraint is essentially the same as a Task (and therefore it already contains the indices property and is compatible with the Subtask type).

Most of the work should be about upgrading OpenSotConstraintAdapter to support the concept of OpenSoT::SubConstraint (mostly replicating the logic inside OpenSotTaskAdapter)

[EnricoMingo]

I think then I took the wrong path since I have tried to replicate the same files as for the Subtask (both in the OpenSoT implementation and base classes)...