Do we want to do this?  I don't think it's a massive problem to have `des` be imported in `galsim.__init__`, but I also think that it's not too much to ask to expect DES-ers to have to add an additional `import galsim.des` statement... You have to do this to access `scipy.stats` for example.

Admittedly, I imagine `galsim.des` is a very fast import, so there's no practical need to separate the too off.  But I can't help but think that having `galsim.des` imported by default makes the whole package *seem* a little more DES-specific than it needs to.  This is purely a personal subjective feeling, of course.  @rmandelb , do you have an opinion?