message: Add mixed_integrator to interface.
diff: #include "coefficient.hpp"
  // PA or MF Operator using libCEED.
  #include "integrator.hpp"
  +// PA Operator supporting mixed finite element spaces.
  +#include "mixed_integrator.hpp"
  // Utility functions
  #include "util.hpp"
  // Wrapper to include <ceed.h>
  