Create the new flexible AST-backed WCS class.
This will be called lsst::afw:;geom::SkyWcs to indicate that it is a celestial WCS. It will implement most of the methods of the existing lsst::afw::image::Wcs. It will subclass from lsst::afw::geom::Transform<Point2Endpoint, SpherePointEndpoint>, because that class already performs the necessary pixel-to-sky transformations. It will include various standard frames to support the LSST pixel convention.
Note that this ticket is purely to create the new WCS class. It is not intended to replace usage of the existing WCS class.