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.

--- a/libinterp/octave-value/ov-base.h
+++ b/libinterp/octave-value/ov-base.h
@@ -583,6 +583,9 @@
   virtual octave_base_value *unique_parent_class (const std::string&)
   { return 0; }
 
+  virtual bool is_instance_of (const std::string&) const
+  { return false; }
+
   virtual octave_function *function_value (bool silent = false);
 
   virtual octave_user_function *user_function_value (bool silent = false);