ToString is const

Former-commit-id: d512894cf4

src/storage/expressions/Expression.cpp

@@ -162,7 +162,7 @@ namespace storm {
             return this->getBaseExpression().accept(visitor);
         }
         
-        std::string Expression::toString() {
+        std::string Expression::toString() const {
             std::stringstream stream;
             stream << *this;
             return stream.str();

src/storage/expressions/Expression.h

@@ -297,7 +297,7 @@ namespace storm {
              *
              * @return The string representation of the expression.
              */
-            std::string toString();
+            std::string toString() const;
             
             friend std::ostream& operator<<(std::ostream& stream, Expression const& expression);