This class attribute helps to set the default in two places. One is for the initialisation in `from_geodetic`, the other is for getting out geodetic coordinates in `to_geodetic`, where I would like the default to be `WGS84` if the coordinates were initialized from geocentric (most likely case), but default to whatever was given in `from_geodetic` if the coordinates were initialized from geodetic coordinates. I thought this was the easiest way to achieve that, but if you think it is cleaner, i can also give it as default in the `from_geodetic` arguments, and set it explicitly in the `from_geocentric` call (am changing this as I'm writing this; see shortly to be updated PR)