Compute force vector for torsion problem with polygonal domains

[carlos-adir]

Following the docs, the force vector F must be computed for torsion problem

$$ F_i = \int_{t_{min}}^{t_{max}} \langle \mathbf{p}, \ \mathbf{p}'\rangle \cdot \ln |\mathbf{r}_i| \ dt $$

[carlos-adir]

$\mathbf{r}_i$ is the distance of the point $\mathbf{p}(t)$ to the source point $\mathbf{s}_i$