Move GCS to variable to shared ParticleDescriptor section

  dcomplex *vkt;

  //! \brief Vector of sizes in units of 2*PI/LAMBDA

  double *vsz;

  //! \brief Total geometric cross-section.

  double gcs;

  // >>> END OF SECTION COMMON TO ALL DESCRIPTOR TYPES <<< //

  // >>> NEEDED BY SPHERE AND CLUSTER <<< //

  const int& ndit = _ndit;

  //! \brief Read-only view of NDM.

  const int& ndm = _ndm;

  //! \brief Total geometric cross-section.

  double gcs;

  //! \brief TBD

  dcomplex *vh;