message: updated double pendulum example
diff: #include "TACSIntegrator.h"
  #include "TACSAssembler.h"
  #include "TACSRigidBody.h"
  +#include "TACSKinematicConstraints.h"
  /*
  Function to test the rigid body dynamics implementation
  @@ -115,9 +116,9 @@ int main( int argc, char *argv[] ){
  //--------------------------------------------------------------------//
  // Create an TACSToFH5 object for writing output to files
  - unsigned int write_flag = (TACSElement::OUTPUT_NODES |
  - TACSElement::OUTPUT_DISPLACEMENTS);
  - TACSToFH5 *f5 = new TACSToFH5(tacs, TACS_RIGID, write_flag);
  + unsigned int write_flag = (TACS_OUTPUT_NODES | TACS_OUTPUT_DISPLACEMENTS);
  + ElementType etype = TACS_BEAM_OR_SHELL_ELEMENT; // How to set rigid type?
  + TACSToFH5 *f5 = new TACSToFH5(tacs, etype, write_flag);
  f5->incref();
  double tinit = 0.0;
  @@ -137,12 +138,10 @@ int main( int argc, char *argv[] ){
  bdf->setPrintLevel(1);
  bdf->setJacAssemblyFreq(1);
  bdf->setUseLapack(0);
  - bdf->setOrderingType(TACSAssembler::NATURAL_ORDER);
  bdf->setOutputFrequency(1);
  // Integrate and write solution to file
  bdf->integrate();
  - bdf->writeSolution("solutionBDF.dat");
  // Delete objects
  bodyA->decref();
  