[solver-constraint] Rename epsilon by energyWeight

[EulalieCoevoet]

When solving an inverse problem, we add a term in the optimization cost function that minimizes the actuation energy. This is achieved by regularizing the QP matrix Q through the addition of epsilon*Waa, where epsilon is selected to be sufficiently small to avoid disrupting the quality of the effector positioning.

Problem:
We named this parameter epsilon, which isn't clear to users.

In this PR:

• Rename epsilon to energyWeight in both the C++ code and user API
• Use the deprecation mechanism for the user API only
• Update the scenes

Also:

• Improve the description of the objective data, set it as read-only, and change its default value to 0
• Remove the deprecated rebuildSystem() method

Needs: SofaDefrost/SoftRobots#344