pass default string by const reference instead of by value

[[Imported from SVN: r4982]]

common/configparser.cc

@@ -182,7 +182,7 @@ string& ConfigParser::operator[] (const string& key)
   }
 }
 
-string ConfigParser::get(const string& key, string defaultValue)
+string ConfigParser::get(const string& key, const string& defaultValue)
 {
   if (hasKey(key))
     return (*this)[key];

common/configparser.hh

@@ -145,7 +145,7 @@ namespace Dune {
      * \param defaultValue default if key does not exist
      * \return value as string
      */
-    std::string get(const std::string& key, std::string defaultValue);
+    std::string get(const std::string& key, const std::string& defaultValue);
 
 
     /** \brief get value as int