InfoSolver: warn if a class are added using _set_attrib instead of _set_child

I made a mistake and it was hard to debug:

    @staticmethod
    def _complete_info_solver(info_solver):
        """Complete the ParamContainer info_solver."""
        OutputCore._complete_info_solver(info_solver)

        classes = info_solver.classes.Output.classes
        classes._set_attrib(
            "PrintStdOut",
            dict(
                module_name="snek5000.output.print_stdout",
                class_name="PrintStdOut",
            ),
        )
        classes._set_attrib(
            "PhysFields",
            dict(module_name="snek5000.output.phys_fields", class_name="PhysFields"),
        )

It is wrong, but passes silently. https://github.com/exabl/snek5000/commit/4a9e7e27c2e71f9ff21f1a779088c89801bb11e3