fix: Reaction calculation

[angelsolaorbaiceta]

Description

Fixes the reaction calculation for the structures package.

Details

The structures.solution.structure class' reaction_for_node() method was incorrectly calculating the reaction force in an externally constrained node like so:

    def reaction_for_node(self, node: StrNodeSolution) -> Vector:
        if not node.is_constrained:
            return Vector(0, 0)

        forces = [bar.force_in_node(node) for bar in self.bars if bar.has_node(node)]

        if node.is_loaded:
            forces.append(node.net_load.opposite())

        return reduce(operator.add, forces)

But, to calculate the reactions correctly, the constrained direction have to be taken into account:

    def reaction_for_node(self, node: StrNodeSolution) -> Vector:
        if not node.is_constrained:
            return Vector(0, 0)

        forces = [bar.force_in_node(node) for bar in self.bars if bar.has_node(node)]

        if node.is_loaded:
            forces.append(node.net_load.opposite())

--      return reduce(operator.add, forces)
++      net_force = reduce(operator.add, forces)

++      # The reaction can only have the components of the constrained directions
++      return Vector(
++          u=net_force.u if node.dx_constrained else 0.0,
++          v=net_force.v if node.dy_constrained else 0.0,
++      )