new octave_value::cell_value method with optional error message * ov.h, ov.cc (octave_value::cell_value): New method. * ov-base.h, ov-base.cc (octave_base_value::cell_value): New default method. * ov-cell.h, ov-cell.cc (octave_cell::cell_value): New method.

%!test
%! a = class_bug44940 ();
%! b = a;
%! c = a ();
%! a.child = 100;
%! assert (a.child, b.child)
%! assert (a.child, c.child)
%! c.child = 500;
%! assert (a.child, b.child)
%! assert (a.child, c.child)