Absolute constraints (ConstraintAbs)

I created absolute constraints for Position, Orientation and Velocity (38a5f8a7)

I called these constraints respectively ConstraintAbsP, ConstraintAbsO, ConstraintAbsV gtests related to these constraints are passing (3d9f904a)

@jsola I thought about creating a ConstraintAbsolute that could handle any of these cases but we need to know the size of the stateblocks in the constructor and... it is different if we consider P/V or Q.

These constraints can used with a FeatureFix. Next step would be to change the name of constraint_fix_3D into ConstraintAbsPose if we want to merge this in master. And if we want to :

• Are we OK with the name of these constraints
• Should I create a specific feature for each of them ? (I think we don't need to...)
• Do we want a constraint for P, Q and V ? If yes then how should we name it ?