Wrapping interpolation for all-sky CAR projection

I'm trying to convert an all-sky CAR projection to HEALPix. Because `reproject_to_healpix` calls `map_coordinates` with `mode='constant'` I get a strip of null pixels at the edges of the map. It would be nice if this method could allow wrapping the interpolation in longitude (e.g. by calling with `mode='wrap'`). I realize wrapping the interpolation doesn't make sense for other projections so maybe you could provide a new method or option that would provide a special treatment for all-sky CAR projections. Note that another issue is how to stop it from wrapping the interpolation in latitude. There I guess the solution is to pad the array passed to `map_coordinates` with one row at low and high latitudes.

Start with reproject_to_healpix and trace its call to map_coordinates. Compare the requested longitude wrapping for all-sky CAR projections with the stated need to avoid latitude wrapping, then define tests that cover edge interpolation and confirm the resulting HEALPix map has no unwanted null strip.

Written by the indexing model from the issue text.