add `make_image` for the galsim fitters

something like this in general

```python
def make_image(self, pars):
    nrow, ncol = self.obs.image.shape
    wcs = self.obs.jacobian.get_galsim_wcs()
    model = self.make_model(pars)
    gsim = model.drawImage(nx=ncol, ny=nrow, wcs=wcs)
    return gsim.array
```