Smoothing kernel is not properly normalized in bkg generation

[samanthalwong]

I can't replicate the sky map artifacts you're seeing (see below; I actually get the opposite where no smoothing has the edge effects but smoothing doesn't), but I believe that the smoothing impact on background rate is due to improper convolution with the Gaussian kernel.

I'll look into better ways to implement some sort of interpolation, whether it be convolving with a Gaussian with proper normalization or spline fitting, which is more similar to ED but also to how HESS smooths their 3D bkgs.

I'll open a different issue for this to keep better track of the actual issue and not my original issue on the wiki, which I haven't run into since we've resolved other issues in gammapy/gammapy-tools. We can reopen if this comes up again.

Originally posted by @samanthalwong in #58 (comment)

[samanthalwong]

Implemented in commit 3cd37f5