OpenSoT::tasks::velocity::Cartesian should have a default id if possible

[arocchi]

In case we control a distal link wrt a base link (i.e., not trying to control an arbitrary point on a body, but using a named frame) the name for the task could be:
6d::base_link::distal_link for pose control
3p::base_link::distal_link for position control
3o::base_link::distal_link for orientation control

In case issue #11 gets implemented by naming a point location on a link first, the automatic naming can be applied always.