Sphericalizing ellipsoids

[asinghvi17]

It's possible to do and Proj does it - we just need to implement that in Core, so that we can go from any datum or ellipsoid to a spherical manifold.

[asinghvi17]

Thanks, just made the change.

BTW, piggybacking on this issue, we should be able to convert a Geodesic() (maybe Ellipsoidal()?) to Spherical() using the same sphericalization parameters as PROJ does for ellipsoids. Here's a link:

        The ellipsoid definition may be augmented with a spherification flag,
    turning the ellipsoid into a sphere with features defined by the ellipsoid.

        Spherification parameters supported are:
            R_A, which gives a sphere with the same surface area as the
            ellipsoid R_V, which gives a sphere with the same volume as the
    ellipsoid

            R_a, which gives a sphere with R = (a + b)/2   (arithmetic mean)
            R_g, which gives a sphere with R = sqrt(a*b)   (geometric mean)
            R_h, which gives a sphere with R = 2*a*b/(a+b) (harmonic mean)

            R_lat_a=phi, which gives a sphere with R being the arithmetic mean
            of of the corresponding ellipsoid at latitude phi.
            R_lat_g=phi, which gives
            a sphere with R being the geometric mean of of the corresponding
    ellipsoid at latitude phi.

            R_C, which gives a sphere with the radius of the conformal sphere
            at phi0.

https://github.com/OSGeo/PROJ/blob/0134c145053ffac92c42b5ab0109b41c8be417e7/src/ell_set.cpp#L51C13-L66C21

https://github.com/OSGeo/PROJ/blob/0134c145053ffac92c42b5ab0109b41c8be417e7/src/ell_set.cpp#L355-L466 is the definition of the spherification function, it doesn't seem too hard.