add SubCombination functionality into ParticleCombination

Phys/ParticleCombiners/include/CombKernel/ParticleCombination.h

@@ -70,6 +70,15 @@ struct ParticleCombination {
     return px * px + py * py + pz * pz;
   }
 
+  template <std::size_t... idxs>
+  ParticleCombination<std::tuple_element_t<idxs, std::tuple<InputTypes...>>...> get_subcombination() const {
+    ParticleCombination<std::tuple_element_t<idxs, std::tuple<InputTypes...>>...> out{m_distance_calculator,
+                                                                                      m_geometry};
+    static_assert( ( ( idxs >= 0 ) && ... ) && ( ( idxs < N ) && ... ) );
+    out.m_children = {m_children[idxs]...};
+    return out;
+  }
+
   /** @brief Magnitude of the four momentum sum of the children.
    */
   Combination::details::fp p() const { return std::sqrt( mom2() ); }