Access to gas column density maps

[cdeil]

As mentioned by @zblz in astropy/astropy#733 (comment) it would be nice to have easy download / interpolation of gas column density maps such as the LAB survey and this tool.

Even better would be code that derives column density maps from the higher-resolution Planck maps (which doesn't distribute such high-level data products as column density yet).

The download of the FITS files should probably by done in a function in gammapy.datasets and then there should be a class that does any computations (if needed) and interpolation ... maybe add as ColumnDensityMap in gammapy.astropy.diffuse?

cc @ellisowen @cnachi

[cdeil]

Access to dust maps is available now in a few other packages.
https://github.com/kbarbary/sfdmap#alternatives

I don't think we want to add / maintain code for this to Gammapy.
Closing this old issue now.

Docs additions explaining the most relevant maps and how to get / use them with Gammapy would still be welcome of course, maybe even code if there are computations needed to go to gamma-ray emission.