nth_coord parameter to axisartist helpers for fixed axis

Helper APIs in axisartist for generating a "fixed" axis on rectilinear axes (FixedAxisArtistHelperRectilinear) no longer take a nth_coord parameter. That parameter is entirely inferred from the (required) loc parameter.

For curvilinear axes, the nth_coord parameter remains supported (it affects the ticks, not the axis position itself), but it is now keyword-only.