Show a warning message when using `reproject_to()`+`assume_spherical_screen()`+`propagate_with_solar_surface()`

[ayshih]

The propagate_with_solar_surface() context manager can be used to account for solar differential rotation when reprojecting a map to a different observation time, but off-disk data are discarded. The assume_spherical_screen() context manager is typically used to preserve off-disk data in reprojections (e.g., for a change in observer location). However, using these these two context managers together does not work as one would hope, and most of the off-disk data are still discarded. Solving that deficiency is a giant mess (tracking under #7173), but in the meantime, we could have reproject_to() emit a warning if it is being called under both contexts.