(fix) Pinned DOF for Neumann boundary condition

Ensures that the same DOF is pinned between two consecutive runs when using Neumann boundary condition

dart/src/wAssign.cpp

@@ -77,9 +77,16 @@ void Initial::write(std::ostream &out) const
 Dirichlet::Dirichlet(std::shared_ptr<tbox::MshData> _msh, int no, int dim, double alpha, double beta, bool pin) : Assign(_msh, no)
 {
     f = new F0PsPhiInf(dim, alpha, beta);
-    // Only retain the first node, so that the DOF associated to this node will be pinned
+    // Only retain one node, so that the DOF associated to this node will be pinned
     if (pin)
-        this->nodes.resize(1);
+    {
+        // Pin node with min x min y min z
+        auto it = std::min_element(nodes.begin(), nodes.end(), [](Node *n1, Node *n2) {
+            return n1->pos[0] < n2->pos[0] || (n1->pos[0] == n2->pos[0] && n1->pos[1] < n2->pos[1]) || (n1->pos[0] == n2->pos[0] && n1->pos[1] == n2->pos[1] && n1->pos[2] < n2->pos[2]);
+        });
+        nodes.resize(1);
+        nodes[0] = *it;
+    }
 }
 Dirichlet::Dirichlet(std::shared_ptr<tbox::MshData> _msh, std::string const &name, int dim, double alpha, double beta) : Assign(_msh, name)
 {