Implemented as a new overload to warpImage that uses a Transform<Point2Endpoint, Point2Endpoint> to describe the transformation.
Once we replace our current Wcs class with lsst::afw::geom::SkyWcs this will be the code that the other overrides call. Until that time users will have to construct the transform manually (which is very easy if one has the SkyWcs for the source and destination exposure).
There is a new test in tests/testWarpExposure.py that tests the code, and a new example examples/timeWarpExposureUsingTransform.py that times the new code. On my computer it runs a bit faster than the old code.