Error and warning messages from pipeline execution currently show only
the class name and pointer. This is usually insufficient to determine
for example which specific algorithm raises the error.

This change introduces an object name for vtkObject. The object name is
stored as a string member ObjectName with setter and getter. It is not a
regular ivar: the MTime is not affected and no ModifiedEvent is invoked.
The objectname is not to be copied by ShallowCopy and DeepCopy
implementations.

A new virtual method on vtkObjectBase, GetObjectDescription, returns the
description that is used in error and warning messages and Print output.
The vtkObjectBase implementation returns the class name and object
pointer. The vtkObject implementation overrides this and prints the
object name as well if it is set.

The error messages issued by vtkExecutive classes now call the
GetObjectDescription method of the algorithm so that if an algorithm has
an object name it appears in the error message.

Added a python test to demonstrate the behavior and a changelog entry to
briefly describe this change.