Meshborn filter (#2925)

Co-authored-by: Paul Romano <paul.k.romano@gmail.com>

include/openmc/particle_data.h

@@ -262,6 +262,10 @@ public:
   int& cell_last(int i) { return cell_last_[i]; }
   const int& cell_last(int i) const { return cell_last_[i]; }
 
+  // Coordinates at birth
+  Position& r_born() { return r_born_; }
+  const Position& r_born() const { return r_born_; }
+
   // Coordinates of last collision or reflective/periodic surface
   // crossing for current tallies
   Position& r_last_current() { return r_last_current_; }
@@ -323,6 +327,7 @@ private:
   int n_coord_last_ {1};  //!< number of current coordinates
   vector<int> cell_last_; //!< coordinates for all levels
 
+  Position r_born_;         //!< coordinates at birth
   Position r_last_current_; //!< coordinates of the last collision or
                             //!< reflective/periodic surface crossing for
                             //!< current tallies

include/openmc/tallies/filter.h

@@ -33,6 +33,7 @@ enum class FilterType {
   MATERIAL,
   MATERIALFROM,
   MESH,
+  MESHBORN,
   MESH_SURFACE,
   MU,
   PARTICLE,

include/openmc/tallies/filter_meshborn.h

@@ -0,0 +1,26 @@
+#ifndef OPENMC_TALLIES_FILTER_MESHBORN_H
+#define OPENMC_TALLIES_FILTER_MESHBORN_H
+
+#include <cstdint>
+
+#include "openmc/position.h"
+#include "openmc/tallies/filter_mesh.h"
+
+namespace openmc {
+
+class MeshBornFilter : public MeshFilter {
+public:
+  //----------------------------------------------------------------------------
+  // Methods
+
+  std::string type_str() const override { return "meshborn"; }
+  FilterType type() const override { return FilterType::MESHBORN; }
+
+  void get_all_bins(const Particle& p, TallyEstimator estimator,
+    FilterMatch& match) const override;
+
+  std::string text_label(int bin) const override;
+};
+
+} // namespace openmc
+#endif // OPENMC_TALLIES_FILTER_MESHBORN_H