Support SN3D normalization

[abhayap]

Hi @jishnub, thanks for providing this library. I've been using it for Ambisonics 3D audio processing. I saw you adding new normalizations. I was wondering if you could add an option for SN3D normalization with no CS phase term? Like the AmbiX format here. Thanks!

[jishnub]

Currently I have only added normalization for the associated Legendre polynomials. Would you like normalization options for these, or for the spherical harmonics?

[abhayap]

Would the normalization of the associated Legendre polynomial apply to the Spherical Harmonics too? The Ambix format needs the normalization on the whole spherical harmonic. More information is [here]:(https://iem.kug.ac.at/fileadmin/media/iem/projects/2011/ambisonics11_nachbar_zotter_sontacchi_deleflie.pdf)