SpherePoint now uses sphgeom vectors

As of DM-13509 lsst::afw::geom::SpherePoint now uses sphgeom vectors instead of lsst::afw::geom::Point3D.

In particular SpherePoint.getVector() returns an lsst::geom::UnitVector3d and SpherePoint can be constructed from an lsst::geom::Vector3d (which is more general than a UnitVector3d, in that its norm need not be 1).

In the rare case that you need an Eigen::Vector3d from SpherePoint::getVector I also added C+±only function lsst::afw:;geom::asEigen which will convert either sphgeom::Vector3d or sphgeom::UnitVector3d to an Eigen::Vector3d.

Note that this eliminated all usa of lsst::afw::geom::Point3D and lsst::afw::geom::Extent3D from the DM stack. Expect a future RFC to get rid of those classes.