Move force-related methods from State to MechanicalState

[ChristianDuriez]

Problem

The methods addToTotalForces() and removeFromTotalForces() are currently defined in the base State class (lines 116-118 in Sofa/framework/Core/src/sofa/core/State.h). However, these methods are mechanics-specific and should not be in the generic State class.

Rationale

• Concept of forces is mechanics-specific: The notion of "force" is inherently tied to mechanical simulations and has no meaning in other physics domains.
• Limits State reusability: Having these methods in the base State class prevents State from being reused for other physics simulations such as:
  • Electrical simulations (where "force" is not applicable)
  • Thermal simulations (heat flow, not forces)
  • Other non-mechanical physics

Suggested Solution

Move the following methods from State to MechanicalState:

• void addToTotalForces(core::ConstVecDerivId forceId) override;
• void removeFromTotalForces(core::ConstVecDerivId forceId) override;

Also move related members:

• AccumulationVecId<TDataTypes, V_DERIV, V_READ> accumulatedForces;
• const AccumulationVecId<TDataTypes, V_DERIV, V_READ>& readTotalForces() const;

This would make the base State class truly generic and extensible for any physics domain, while keeping mechanical-specific functionality in MechanicalState.

Impact

This is a refactoring that would improve the architecture's extensibility for non-mechanical physics simulations while maintaining backward compatibility through MechanicalState.