Refactor tasks and fix Draco 3 example

[stephane-caron]

• Fix Draco 3 example
• Cost vector is now defined for all tasks
• Levenberg-Marquardt damping is now defined for all tasks, with a default to zero

[stephane-caron]

@ymontmarin This PR is ready for review. :detective:

[stephane-caron]

Thank you for checking :smiley:

I can do it if you think it is useful for readabillity of the code

Currently it's readable but a bit dumb :sweat_smile: It fits the fact that some child classes add their own attributes to the repr (e.g. frame task adding its target).

If auto repr generation is simple enough to maintain and reduces the amount of boilerplate code I'm all for it :+1: