Specify for expression functions if they are static

... and prepare them whenever possible

python/core/qgsexpression.sip

@@ -570,12 +570,31 @@ Does this function use a geometry object.
 %Docstring
  True if this function should use lazy evaluation.  Lazy evaluation functions take QgsExpression.Node objects
  rather than the node results when called.  You can use node->eval(parent, feature) to evaluate the node and return the result
- Functions are non lazy default and will be given the node return value when called **
+ Functions are non lazy default and will be given the node return value when called.
  :rtype: bool
 %End
 
         virtual bool isStatic( const QgsExpression::NodeFunction *node, QgsExpression *parent, const QgsExpressionContext *context ) const;
 %Docstring
+ Will be called during prepare to determine if the function is static.
+ A function is static if it will return the same value for every feature with different
+ attributes and/or geometry.
+
+ By default this will return true, if all arguments that have been passed to the function
+ are also static.
+
+.. versionadded:: 3.0
+ :rtype: bool
+%End
+
+        virtual bool prepare( const QgsExpression::NodeFunction *node, QgsExpression *parent, const QgsExpressionContext *context ) const;
+%Docstring
+ This will be called during the prepare step() of an expression if it is not static.
+
+ This can be used by functions to do any preparation steps that might help to speedup the upcoming
+ evaluation.
+
+.. versionadded:: 3.0
  :rtype: bool
 %End
 
@@ -828,8 +847,6 @@ return index of the function in Functions array
         virtual QgsExpression::Node *clone() const = 0;
 %Docstring
  Generate a clone of this node.
- Make sure that the clone does not contain any information which is
- generated in prepare and context related.
  Ownership is transferred to the caller.
 
  :return: a deep copy of this node.
@@ -888,6 +905,16 @@ return index of the function in Functions array
  :rtype: bool
 %End
 
+
+      protected:
+
+        void cloneTo( QgsExpression::Node *target ) const;
+%Docstring
+ Needs to be called by all subclasses as part of their clone() implementation.
+
+.. versionadded:: 3.0
+%End
+
       private:
         virtual bool prepareNode( QgsExpression *parent, const QgsExpressionContext *context ) = 0 ;
 %Docstring