[RodStraightSection] Fix rest shape double-offset for non-first sections (sofa-framework#223) (sofa-framework#224)

x_used is the global curvilinear abscissa; adding x_start again placed
the rest node at 2*x_start + local_offset instead of x_used. For wires
with two or more RodStraightSections this produced enormous elastic
restoring forces at every timestep, causing simulation explosion.

Fix: use x_used directly, consistent with RodSpireSection and RodMeshSection.

Co-authored-by: Lennart Karstensen <lennart.karstensen@ipa.fraunhofer.de>

Author: lkarstensen

src/BeamAdapter/component/model/RodStraightSection.inl

@@ -53,7 +53,7 @@ bool RodStraightSection<DataTypes>::initSection()
 template <class DataTypes>
 void RodStraightSection<DataTypes>::getRestTransformOnX(Transform& global_H_local, const Real x_used, const Real x_start)
 {
-    global_H_local.set(type::Vec3(x_start + x_used, 0.0, 0.0), Quat());
+    global_H_local.set(type::Vec3(x_used, 0.0, 0.0), Quat());
 }