ENH: allow sph_inkn for order n=0

The modified spherical Bessel functions ``sph_in`` and ``sph_kn`` allow for order ``n=0`` while the combined function ``sph_inkn`` does not. In ``specfun``, the minimum order has to be ``n=1`` which is solved in ``sph_in`` and ``sph_kn`` by using at least order ``n=1`` and returning a smaller array if ``n=0`` was requested. I propose to do the same for ``sph_inkn`` to avoid an unnecessary ``specfun.error``.