Add normalization routines

[strawpants]

Currently the spherical harmonic data are real, and 4-pi normalized, which are commonly used in Geodesy. To facilitate working with other normalizations, the module could be improved by:

1. Keep track on the normalization used as a Xarray attribute (these will also be saved to files and thus serve as documentation)
2. Add conversion routines to the Xarray accessor to facilitate the conversion between different normalizations
3. Make the backends aware of the used normalization and use accordingly