Implement "isa" for classdef objects. * ov.h (octave_value::is_instance_of): New method. * ov-base.h (octave_base_value::is_instance_of): Likewise. * ov-class.h (octave_class::is_instance_of): New method declaration. * ov-class.cc (octave_class::is_instance_of): Implement it with find_parent_class. * ov-classdef.h (octave_classdef::is_instance_of): New method declaration. * ov-classdef.cc (octave_classdef::is_instance_of): Implement it with lookup_class and is_superclass.