STYLE: GetStopConditionDescription abstract method

Every derivative class of itkObjectToObjectOptimizerBase
had a GetStopConditionDescription function defined and
making it an abstract method so that it is accessible to
the SimpleITK layer consistently.

virtual const StopConditionReturnStringType
GetStopConditionDescription() const = 0;

Change-Id: Ic37b656b677be7c153dd2ec3df88a7d37949a376
Merge topic 'AddAbstractBaseMethodForOptimizer'

e658eb6b STYLE: GetStopConditionDescription abstract method