Add Numba bindings

[Konjkov]

Hello sphericart developers.

I need Numba binding of your library. I think it is not difficult to implement.

the implementation is similar to
https://github.com/Konjkov/pycasino/blob/sphericart/casino/harmonics_new.py

Vladimir.

[ceriottm]

We'd be very happy to review a PR adding numba bindings, looks like you're already half-way done.