Npred cube2

Implementation which I intend to use to produce significance images (currently this is still running on my machine).

The unit tests will need much improvement soon as I just implemented some quick ones. Looking at the outputs with ds9 using the FermiGalacticCenter files seemed reasonable, but obviously not rigorous yet.

Don't know if you were aiming to merge this asap, so did a pull request just in case.
I see this test error locally:
https://gist.github.com/cdeil/ca385fe7271a6a9bfdb2

Looks like the signature is
```
def convolve_npred_cube(npred_cube, psf_object, max_offset, resolution=1):
```
i.e. you have to pass a `psf` object?

I don't know why this didn't show up on travis-ci ... can you rebase this PR on master to make sure that's not the issue?