I wonder if this piece of information, namely that a `GSObject... is naturally defined in world coordinates` ought to be repeated or emphasized elsewhere in the documentation.

It also perhaps clashes a little with what the user might assume from a cursory reading of the documentation to (for example) the `GSObject` method docstring for `applyTransformation()`, due solely to the choice of notation used:
```
        """Apply a transformation to this object defined by an arbitrary Jacobian matrix.

        This applies a Jacobian matrix to the coordinate system in which this object 
        is defined.  It changes a profile defined in terms of (x,y) to one defined in 
        terms of (u,v) where:

            u = dudx x + dudy y
            v = dvdx x + dvdy y

        That is, an arbitrary affine transform, but without the translation (which is 
        easily effected via applyShift).
```
It's the (x,y) -> (u,v) here that I think might cause a little confusion for readers given the (correct I think for our purposes) underlying framework that all GSObjects are defined in world coordinates.

I'm not sure how best to tackle this, but the WCS changes here do in fact mean that there has been a rather fundamental shift in the way we think about coordinate systems in GalSim, and I wonder how best to present that.  I'm going to carry on reading and think a bit more...