[FEM] Implement getPotentialEnergy for Linear & Corotational FEMForceField

[th-skam]

Implements getPotentialEnergy() for CorotationalFEMForceField and LinearSmallStrainFEMForceField components. Both now compute elastic strain energy using the form:

$$E = \sum_e \frac{1}{2}\mathbf{u_e}^T\mathbf{K_e}\mathbf{u_e}$$

• $\mathbf{u_e}$: element displacement of DOFs
• $\mathbf{K_e}$: element stiffness matrix

The corotational form uses the local displacements and so gives the energy due to local deformations.