abstraction: allow constant retrieval of value from value holder

alib2abstraction/src/abstraction/ValueHolder.hpp

@@ -23,6 +23,10 @@ public:
 	Type && getValue ( ) override {
 		return std::move ( m_data.value ( ) );
 	}
+
+	const Type & getValue ( ) const override {
+		return m_data.value ( );
+	}
 };
 template < class Type >
@@ -39,6 +43,10 @@ public:
 	Type && getValue ( ) override {
 		return std::move ( m_data->get ( ) );
 	}
+
+	const Type & getValue ( ) const override {
+		return m_data->get ( );
+	}
 };
 template < class Type >

alib2abstraction/src/abstraction/ValueHolderInterface.hpp

@@ -15,6 +15,8 @@ public:
 	virtual Type && getValue ( ) = 0;
+	virtual const Type & getValue ( ) const = 0;
+
 };
 template < class ParamType >