Fix A Couple of Compiler and Static Code Analysis Warnings

In particular

  * Tag a single argument constructor as 'explicit',
  * Remove an unused private function
  * Fix mismatched tags (struct vs. class) in forward declaration
  * Return 'false' in an impossible updateHyst() case

While here, also use a real UnitSystem object instead of creating
a METRIC system just to infer unit strings.

tests/test_Summary_Group.cpp

@@ -228,8 +228,8 @@ struct setup
 
     // ------------------------------------------------------------------------
 
-    setup(std::string        case_name,
-          const std::string& path = "UDQ_ACTIONX_TEST1_U.DATA")
+    explicit setup(std::string        case_name,
+                   const std::string& path = "UDQ_ACTIONX_TEST1_U.DATA")
         : deck     { Parser{}.parseFile(path) }
         , es       { deck }
         , grid     { es.getInputGrid() }