[CL] Pass database as const&. Avoids string copy.

ChemistryLib/PhreeqcKernel.cpp

@@ -26,7 +26,7 @@ PhreeqcKernel::PhreeqcKernel(
     std::size_t const num_chemical_systems,
     std::vector<std::pair<int, std::string>> const&
         process_id_to_component_name_map,
-    std::string const database,
+    std::string const& database,
     AqueousSolution aqueous_solution,
     std::unique_ptr<EquilibriumReactants>&& equilibrium_reactants,
     std::unique_ptr<Kinetics>&& kinetic_reactants,

ChemistryLib/PhreeqcKernel.h

@@ -35,7 +35,7 @@ public:
     PhreeqcKernel(std::size_t const num_chemical_systems,
                   std::vector<std::pair<int, std::string>> const&
                       process_id_to_component_name_map,
-                  std::string const database,
+                  std::string const& database,
                   AqueousSolution aqueous_solution,
                   std::unique_ptr<EquilibriumReactants>&& equilibrium_reactants,
                   std::unique_ptr<Kinetics>&& kinetic_reactants,