[CL] Phreeqc; Move global variable into class.

The instance_id is used exclusively in the PhreeqcIO class.

ChemistryLib/PhreeqcIO.cpp

@@ -29,8 +29,6 @@ std::ostream& operator<<(std::ostream& os,
               std::ostream_iterator<DataBlock>(os));
     return os;
 }
-
-static const int phreeqc_instance_id = 0;
 }  // namespace
 
 PhreeqcIO::PhreeqcIO(

ChemistryLib/PhreeqcIO.h

@@ -71,5 +71,6 @@ private:
     std::vector<std::pair<int, std::string>> const&
         _process_id_to_component_name_map;
     double _dt = std::numeric_limits<double>::quiet_NaN();
+    const int phreeqc_instance_id = 0;
 };
 }  // namespace ChemistryLib