I didn't see any unit tests that check this new function.  It should be sufficient to just check that `obj.applyLensing(g1,g2,mu)` draws the same image as `obj.applyShear(g1,g2); obj.applyMagnification(sqrt(mu))`.  

Which raises the question: Should we regularize the sqrt?  The `applyMagnification` function assumes a linear scale to match `applyDilation` (for which I think the linear factor is intuitive).  Should we change this to accept a "mu" value rather than a "scale" value to match the more lensing-typical meaning?