It doesn't now, but that's not a bad idea.  Should we emit the warning if the FitsWCS function is called implicitly from `im = galsim.fits.read(file)` too?  Or just if the user directly uses `wcs = galsim.FitsWCS(file)`? 

I'm not sure if it would be more annoying to get the warnings when you don't care about the wcs in the header, or to not get the warnings when you thought it should read the wcs.  Note: the `read` function only uses `FitsWCS` if there is at least a `CTYPE1` keyword, so files with no WCS information at all would not get a warning in any case.