IO/config: return constant references whenever possible

Author: andrea-iob

src/IO/configuration_config.cpp

@@ -151,7 +151,7 @@ bool Config::hasOption(const std::string &key) const
     \param key is the name of the option
     \result The specified option.
 */
-std::string Config::get(const std::string &key) const
+const std::string & Config::get(const std::string &key) const
 {
     return m_options->at(key);
 }

src/IO/configuration_config.hpp

@@ -58,7 +58,7 @@ class Config
     Options & getOptions();
     const Options & getOptions() const;
     bool hasOption(const std::string &key) const;
-    std::string get(const std::string &key) const;
+    const std::string & get(const std::string &key) const;
     std::string get(const std::string &key, const std::string &fallback) const;
     void set(const std::string &key, const std::string &value);
     bool removeOption(const std::string &key);