IO/config: return constant references whenever possible

Author: andrea-iob

src/IO/configuration_config.cpp

@@ -213,7 +213,7 @@ void Config::set(const std::string &key, const std::string &value)
     \param name is the name of the attribute
     \result The value of the specified attribute.
 */
-std::string Config::getAttribute(const std::string &key, const std::string &name) const
+const std::string & Config::getAttribute(const std::string &key, const std::string &name) const
 {
     return getOption(key).attributes.at(name);
 }

src/IO/configuration_config.hpp

@@ -67,7 +67,7 @@ class Config
     std::string get(const std::string &key, const std::string &fallback) const;
     void set(const std::string &key, const std::string &value);
 
-    std::string getAttribute(const std::string &key, const std::string &name) const;
+    const std::string & getAttribute(const std::string &key, const std::string &name) const;
     std::string getAttribute(const std::string &key, const std::string &name, const std::string &fallback) const;
     void setAttribute(const std::string &key, const std::string &name, const std::string &value);