Updated blib.py and psf-fit-overlays.py

There's still a problem that I have with fitting. The psf's are poorly
sampled, since they are small in comparison to the FoV. This leads to a
mismatching when calculating the FWHM. I think the solution to this is
the following:
- Use the im toolbox to apply weights to the visibilities (im.weight)
- Define the image with im.defineimage()
- Calcute the psf (im.approximate)