Fix DistanceGridCollisionModel

[epernod]

Fix crash and make scenes work as expected

[th-skam]

In order to use the RigidDistanceGridCollisionModel with a FreeMotionAnimationLoop, a MechanicalObject with a Rigid3d template must be defined in the Node. The model expects a Rigid3d MechanicalState:
https://github.com/sofa-framework/sofa/blob/1eb5242037be574a278b3b6621674a94fd7b8aa3/applications/plugins/SofaDistanceGrid/src/SofaDistanceGrid/components/collision/DistanceGridCollisionModel.h#L117
Without defining a MechanicalObject in the node (as is the case in the example scene of the repo) or defining one that defaults to Vec3d, this pointer is empty and causes a crash.

Full bug tracing rabbit hole:

• The crash is caused by this line when creating a Lagrangian constraint for the contact point:
  https://github.com/sofa-framework/sofa/blob/1eb5242037be574a278b3b6621674a94fd7b8aa3/Sofa/Component/Constraint/Lagrangian/Model/src/sofa/component/constraint/lagrangian/model/BaseContactLagrangianConstraint.inl#L73
  It attempts to access the free position coordinates of the Mech State dofs but the vector is empty.

• This is due to the mappers, which fail to update the free position
  https://github.com/sofa-framework/sofa/blob/1eb5242037be574a278b3b6621674a94fd7b8aa3/Sofa/Component/Collision/Response/Contact/src/sofa/component/collision/response/contact/BaseUnilateralContactResponse.inl#L195
  because the mapper is actually null and the update never happens:
  https://github.com/sofa-framework/sofa/blob/1eb5242037be574a278b3b6621674a94fd7b8aa3/Sofa/Component/Collision/Response/Mapper/src/sofa/component/collision/response/mapper/RigidContactMapper.h#L110-L118

• and the mapper is null because our collision model returns an empty MechanicalState that redirects flow to the else statement that sets mapping to nullptr.
  https://github.com/sofa-framework/sofa/blob/1eb5242037be574a278b3b6621674a94fd7b8aa3/Sofa/Component/Collision/Response/Mapper/src/sofa/component/collision/response/mapper/RigidContactMapper.inl#L56

Note

This is not the case when using the DefaultAnimationLoop because the problem is caused during the creation of Lagrangian constraints as contact points.

A bit messy but perhaps there was a reason for this approach. Problems:

• User is never notified
• Sneaky little differences in setup between Default and FreeMotion animation loops that might confuse users

[th-skam]

Using a MechanicalObject templated on Rigid3d introduces other problems in the RigidDistanceGridCollisionModel.
For instance, in the example scenes of the plugin that work with the DefaultAnimationLoop, by defining a Rigid3d MechanicalObject, this block of code is activated:
https://github.com/sofa-framework/sofa/blob/1eb5242037be574a278b3b6621674a94fd7b8aa3/applications/plugins/SofaDistanceGrid/src/SofaDistanceGrid/components/collision/DistanceGridCollisionModel.cpp#L188
which simply grabs the dofs (coordinates) and assigns them to a translation variable.
As a result, the collision model is translated at t=0: