[BL] Add getConfigAttribute with default value.

Update documentation mismatches in other getConfigAttribute*().

BaseLib/ConfigTree-impl.h

@@ -187,6 +187,16 @@ getConfigAttribute(std::string const& attr) const
     error("Did not find XML attribute with name \"" + attr + "\".");
 }
 
+template <typename T>
+T ConfigTree::getConfigAttribute(std::string const& attr,
+                                 T const& default_value) const
+{
+    if (auto a = getConfigAttributeOptional<T>(attr))
+        return *a;
+
+    return default_value;
+}
+
 template<typename T>
 boost::optional<T>
 ConfigTree::

BaseLib/ConfigTree.h

@@ -364,18 +364,32 @@ public:
 
     /*! Get XML attribute \c attr of type \c T for the current parameter.
      *
-     * \return the requested attribute
+     * \return the requested attribute's value.
      *
-     * \pre \c param must not have been read before from this ConfigTree.
+     * \pre \c attr must not have been read before from the current parameter.
      */
     template<typename T> T
     getConfigAttribute(std::string const& attr) const;
 
-    /*! Get XML attribute \c attr of type \c T for the current parameter if present.
+    /*! Get XML attribute \c attr of type \c T for the current parameter or the
+     * \c default_value.
      *
-     * \return the requested attribute
+     * This method has a similar behaviour as getConfigAttribute(std::string
+     * const&) except in case of errors the \c default_value is returned.
      *
-     * \pre \c param must not have been read before from this ConfigTree.
+     * \return the requested attribute's value.
+     *
+     * \pre \c attr must not have been read before from the current parameter.
+     */
+    template <typename T>
+    T getConfigAttribute(std::string const& attr, T const& default_value) const;
+
+    /*! Get XML attribute \c attr of type \c T for the current parameter if
+     * present.
+     *
+     * \return the requested attribute's value.
+     *
+     * \pre \c attr must not have been read before from the current parameter.
      */
     template<typename T> boost::optional<T>
     getConfigAttributeOptional(std::string const& attr) const;