Possible Octave interpreter error

[kolmanthomas]

With the changes in #23, some surface-level issues are fixed but now tons of tests in tests/chebop are aborting with the same error:

error: octave_base_value::parent_class_name_list(): wrong type argument 'object'

Not sure exactly what the problem is here.

[kolmanthomas]

I investigated this issue a bit futher and it ties back to the fact that cat, horzcat and vertcat are not defined for classdef arrays. We don't see the same issue with Chebfun objects because they override horzcat and vertcat, but classes like treeVar and adchebfun don't.