include or link to dependency `axes_extremal_xyz.m`

[johnyf]

This dependency can be found at: https://github.com/johnyf/plot_utils/blob/bd304279ee0e000f72fc936e0ef9f3a690d4e654/plot_scalings/axes_extremal_xyz.m. Its location appears to be undocumented in the README. Thanks to Nick Fry and others that have mentioned this at the FEX page.

Required at various places:

https://github.com/johnyf/fig2u3d/blob/74fe75d424665e3a8194337036f923962620bfd5/fig2u3d.m#L104 https://github.com/johnyf/fig2u3d/blob/74fe75d424665e3a8194337036f923962620bfd5/view2vws.m#L14 https://github.com/johnyf/fig2u3d/blob/74fe75d424665e3a8194337036f923962620bfd5/fig2idtf/auxiliary/create_marker_lines.m#L10