Improve VirtualRobot::RuntimeEnvironment::getValue

* the templated version now works withstrings via if constexpr
* add version taking an output parameter

VirtualRobot/RuntimeEnvironment.h

@@ -99,20 +99,32 @@ namespace VirtualRobot
         template<class T>
         static T getValue(const std::string& key, const T& defaultValue)
         {
-            static_assert(std::is_arithmetic_v<T>);
-            try
+            if constexpr(std::is_same_v<T, std::string>)
             {
-                return hasValue(key) ? boost::lexical_cast<T>(getValue(key)) : defaultValue;
+                return getValue(key, defaultValue);
             }
-            catch(...)
+            else
             {
-                VR_WARNING << "Failed to lexical cast value '" << getValue(key)
-                           << "' for key '" << key << "' to "
-                           << boost::core::demangle(typeid(T).name())
-                           << "'" << std::endl;
-                throw;
+                static_assert(std::is_arithmetic_v<T>);
+                try
+                {
+                    return hasValue(key) ? boost::lexical_cast<T>(getValue(key)) : defaultValue;
+                }
+                catch(...)
+                {
+                    VR_WARNING << "Failed to lexical cast value '" << getValue(key)
+                               << "' for key '" << key << "' to "
+                               << boost::core::demangle(typeid(T).name())
+                               << "'" << std::endl;
+                    throw;
+                }
             }
         }
+        template<class T>
+        static void getValue(T& target, const std::string& key, const auto& defaultValue)
+        {
+            target = getValue<T>(key, defaultValue);
+        }
         static bool hasValue(const std::string& key);
 
         /// Indicate whether the given flag was specified.