Uses Aplpy FITSFigure in Npred plots

Still a few things to do here (I haven't figured out how to do these yet).
- [ ] Add colourbars to the plots
- [ ] Use better colour scale for the background ratio image
- [x] Add titles to each of the subplots

Anything else?
It's OK ... no need to track this down.

One thing I noticed was that the gammapy npred image was very incorrect at the edges ... it's better to use a PSF convolve mode that e.g. mirrors the values at the edges instead of padding with zero ... this way one gets roughly correct results at the edges.