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
- [ ] Add titles to each of the subplots
Anything else?
I fixed this to 'MeV'. Previously the units were wrong, but I didn't notice as they ignored and 'MeV' was assumed. The fix I made the other day meant these units were no longer ignored, so I correct this here.