Idyntree: Document semantics difference between Pinocchio quantities and iDynTree KinDynComputations quantities

[traversaro]

Things to check (see https://github.com/robotology/idyntree/issues/673#issuecomment-615168900) :

• DOFs and link serializations
• Velocity, force and acceleration representations for the base quantities
• How we need to transform the vector and matrix input and output quantities appropriately given these differences

[traversaro]

cc @GiulioRomualdi

[traversaro]

Related issues:

• https://github.com/stack-of-tasks/pinocchio/issues/1171
• https://github.com/stack-of-tasks/pinocchio/issues/1140

[GiulioRomualdi]

Before I forgot I want to drop here this table of conversion:

| iDynTree | Pinocchio |
|:-----------------------------:|:-------------------:|
| MIXED_REPRESENTATION | LOCAL_WORLD_ALIGNED |
| INERTIAL_FIXED_REPRESENTATION | WORLD |
| BODY_FIXED_REPRESENTATION | LOCAL |

[DanielePucci]

CC @robotology/iit-dynamic-interaction-control