make concatenation of class objects work * data.h: New file. * src/Makefile.am (octinclude_HEADERS): Add it to the list. * data.cc (attempt_type_conversion): New static function. (do_class_concat): New function. (do_cat): Use it if any elements of the list are objects. Check whether any elements of the list are objects or cells. Check whether all elements of the list are complex. Check whether the first element of the list is a struct. Maybe convert elements of the list to cells. New tests for horzcat and vertcat. * data.h (do_class_concat): Provide decl. * ov-class.h (octave_class::octave_class): Allow optional parent list. * ov.h, ov.h (octave_value::octave_value (const Octave_map&, const std::string&)): Likewise. * pt-mat.cc (do_class_concat): New static function. (tree_matrix::rvalue1): Use it to concatenate objects.