Filter weight implementation (#3345)

Co-authored-by: Grego01-biot <grego01@pc-neutronic-06.psfc.mit.edu>
Co-authored-by: Patrick Shriwise <pshriwise@gmail.com>
Co-authored-by: Paul Romano <paul.k.romano@gmail.com>

include/openmc/tallies/filter.h

@@ -44,6 +44,7 @@ enum class FilterType {
   SURFACE,
   TIME,
   UNIVERSE,
+  WEIGHT,
   ZERNIKE,
   ZERNIKE_RADIAL
 };

include/openmc/tallies/filter_weight.h

@@ -0,0 +1,51 @@
+#ifndef OPENMC_TALLIES_FILTER_WEIGHT_H
+#define OPENMC_TALLIES_FILTER_WEIGHT_H
+
+#include <string>
+
+#include "openmc/span.h"
+#include "openmc/tallies/filter.h"
+#include "openmc/vector.h"
+
+namespace openmc {
+
+//==============================================================================
+//! Bins the weights of the particles.
+//==============================================================================
+
+class WeightFilter : public Filter {
+public:
+  //----------------------------------------------------------------------------
+  // Constructors, destructors
+
+  ~WeightFilter() = default;
+
+  //----------------------------------------------------------------------------
+  // Methods
+
+  std::string type_str() const override { return "weight"; }
+  FilterType type() const override { return FilterType::WEIGHT; }
+
+  void from_xml(pugi::xml_node node) override;
+
+  void get_all_bins(const Particle& p, TallyEstimator estimator,
+    FilterMatch& match) const override;
+
+  void to_statepoint(hid_t filter_group) const override;
+
+  std::string text_label(int bin) const override;
+
+  //----------------------------------------------------------------------------
+  // Accessors
+
+  const vector<double>& bins() const { return bins_; }
+  void set_bins(span<const double> bins);
+
+protected:
+  //----------------------------------------------------------------------------
+  // Data members
+  vector<double> bins_;
+};
+
+} // namespace openmc
+#endif // OPENMC_TALLIES_FILTER_WEIGHT_H