[PROJ] Determine if same celestial body, prior to 8.1

[Nyall Dawson]

Hi list,

I'm wondering if there's any way in proj versions < 8.1 to determine
(from the C api) whether two ellipsoid objects refer to the same
celestial body?

Ellipsoid::celestialBody() wasn't exposed to C api prior to 8.1, and
that's the method used by proj internally in
createOperationsGeodToGeod to determine whether two ellipsoids belong
to the same body.

Nyall