Notebook tutorial that demonstrates how to convert between healpix alms and s2fft convention

[jasonmcewen]

We should include a notebook tutorial that demonstrates how to convert between the spherical harmonic coefficient conventions used by HEALPix (i.e. healpy) and s2fft.

Conversion functions are documented here.

But since this is likely to be a very common use case it would be useful to have a notebook demonstrating this.