The main reason I did this (originally for the des_shapelets and des_psfex modules) was that I couldn't figure out a way to get the config versions to work without the des module being imported by default.  If you can think of a way to get `psf : type : DES_Shapelet` to work without the `from . import des` in `galsim/__init__.py`, then I'm fine with requiring an explicit `import galsim.des` in python.