Make micro_photon_xs available on C++ side

include/openmc/photon.h

@@ -22,16 +22,35 @@ public:
   int Z_; //! Atomic number
 };
 
+//==============================================================================
+//! Cached microscopic photon cross sections for a particular element at the
+//! current energy
+//==============================================================================
+
+struct ElementMicroXS {
+  int index_grid; //!< index on element energy grid
+  double last_E {0.0}; //!< last evaluated energy in [eV]
+  double interp_factor; //!< interpolation factor on energy grid
+  double total; //!< microscopic total photon xs
+  double coherent; //!< microscopic coherent xs
+  double incoherent; //!< microscopic incoherent xs
+  double photoelectric; //!< microscopic photoelectric xs
+  double pair_production; //!< microscopic pair production xs
+};
+
 //==============================================================================
 // Global variables
 //==============================================================================
 
 namespace data {
-
 extern std::vector<PhotonInteraction> elements;
-
 } // namespace data
 
+namespace simulation {
+extern ElementMicroXS* micro_photon_xs;
+#pragma omp threadprivate(micro_photon_xs)
+} // namespace simulation
+
 } // namespace openmc
 
 #endif // OPENMC_PHOTON_H