Behaviour of precompute transforms with `kernel = None` default unclear

[matt-graham]

Both the forward and inverse precompute transforms default to a value of None for their kernel argument and the corresponding docstrings indicate this argument is optional:

https://github.com/astro-informatics/s2fft/blob/7cf80bb87201fbdf39a1531a31b25a3112c1f140/s2fft/precompute_transforms/spherical.py#L14-L23

https://github.com/astro-informatics/s2fft/blob/7cf80bb87201fbdf39a1531a31b25a3112c1f140/s2fft/precompute_transforms/spherical.py#L289-L298

Looking through the underlying implementations though, I cannot spot any logic for handling a kernel is None case with the kernel value passed as is in to a call to einsum.

I think we probably either want to remove the None defaults (and optional labels in the docstrings) or add some logic for automatically computing an appropriate value for kernel if left as the default None?