perf(engine_multiscale): performance enhancments due to improved hashing, locality, and data structure optimization

This particular commit speeds up QSE solving for systems where reverse reactions and engine caching is disabled by about 24%

src/include/gridfire/partition/composite/partition_composite.h

@@ -90,6 +90,9 @@ namespace gridfire::partition {
      private:
          quill::Logger* m_logger = fourdst::logging::LogManager::getInstance().getLogger("log");
          std::vector<std::unique_ptr<PartitionFunction>> m_partitionFunctions; ///< Set of partition functions to use in the composite partition function.
+
+         mutable std::unordered_map<uint_fast32_t, const PartitionFunction&> m_supportCache; ///< Cache mapping isotope keys to supporting partition functions for fast lookup.
+
      private:
         /**
          * @brief Instantiate a sub-function by its type.