iCub3 fixed_feet model broken

[GiulioRomualdi]

iCub3 does not have r_foot and l_foot links so the following does not work

icub-models/feet_fixed_model.sdf.in

Lines 8 to 16 in 4bf86ba

	<joint name="fixed_left" type="fixed">
	<parent>world</parent>
	<child>iCub::l_foot</child>
	</joint>
	<joint name="fixed_right" type="fixed">
	<parent>world</parent>
	<child>iCub::r_foot</child>
	</joint>

As a consequence, the feet_fixed model is not fixed

[traversaro]

This is not the first time it happens, we should look if there is any way to load the models and check if such error arise, to incorporate it in CI. Having said that, for this specific problem probably we can add a feet_fixed_model_icub3.sdf.in and for iCub3 models parse that one instead?