I think I'd prefer a simpler structure here where buildGrid always builds the shear and kappa grids.  It's really not that much extra computation.  Basically one more fft.  The fft scales as Nln(N) where N is the number of total grid points (nx * ny).  Typically N is equal to (or similar to) the number of galaxies we're going to draw on the image, and we have a bunch of other things that scale with this N with much larger prefactors.

So I suggest having this function be buildGrid, not buildGriddedShears, and have it do both shear and kappa.  It can return all three, or you can add getGriddedShear and getGriddedKappa functions that can be used after this function is called.  Either way.