Provide mathematical definition for components::WorldLinearAcceleration and components::LinearAcceleration

[traversaro]

The definition for the components components::WorldLinearAcceleration and components::LinearAcceleration currently provide in https://github.com/ignitionrobotics/ign-gazebo/blob/de5b025968c9cf0cfbfd8a852458482e87c70c6c/include/ignition/gazebo/components/LinearAcceleration.hh may be confusing:

• LinearAcceleration : A component type that contains linear acceleration of an entity represented by ignition::math::Vector3d.
• WorldLinearAcceleration : A component type that contains linear acceleration of an entity
  /// in the world frame represented by ignition::math::Vector3d.

In particular, for this description it is not clear for example in which frame the time derivative is performed, an what are the relation between this components, the corresponding velocity components, and the quite common in robotics definition of acceleration according to Featherstone books.

As far as I am able to read from the code, it seems to be that, if we define:

• ʷoₗ ∈ ℝ³ : the 3d vector that connects the origin of the world frame w and the origin of the link frame l, that points toward the origin of frame l and oriented with the orientation of world frame w
• ʷRₗ ∈ SO(3) : the rotation matrix that multiplied by a 3d vector expressed with orientation of the l frame it transform it in a 3d vector oriented according to the w frame
• ʷωₗ ∈ ℝ³ is the world angular velocity of frame L, defined such that ʷωₗ × = ʷṘₗ ʷRₗᵀ

Then we have:

Name	Definition
components::LinearVelocity	ʷRₗᵀ ʷȯₗ
components::WorldLinearVelocity	ʷȯₗ
components::LinearAcceleration	ʷRₗᵀ ʷöₗ
components::WorldLinearAcceleration	ʷöₗ

It this is correct, it may be useful to clearly specify that components::LinearAcceleration is not the derivative of components::LinearVelocity, and that components::LinearAcceleration is not the linear part of the 6D Body Spatial Acceleration, that are both aspects that in my experience can confuse users.

This is related to #83, in particular for the Physics system.

cc @diegoferigo