FEM simulation  setup
We can simulate truss edge deformation if the truss has some fixed nodes and are exposed to one or more external forces
We define fixed nodes by using an array describing if each node is constrained (fixed) in each xyz direction
 External constraints array
 One node for each row, each node can be fixed (0) of free to move during simulation (1) in each xyz direction
 Variable name  extC
We specify external force in each node by an array of xyz force vectors, where each row corresponds to each node in N
 External force array
 One node force vector in each row corresponding to each node in N
 Variable name  extF
In the figure we see the force in the x direction on the top node. Node 1 is constrained in the zdirection, node 2,4 is constrained in all directions.
The 3 constrained nodes are illustrated with red dots
