Calculate emission in certain rings

[tospines]

Hi!

It would be useful for certain analyses, specially those having to do with gamma rays, to have a function that allows the user to compute the integrated emission along the line-of-sight for certain rings around the Galactic center. This can be easily done, for example, having an array of values of normalization that multiply the gas density in each of the rings in the Ring Model. This is already implemented for the molecular hydrogen with the XCO factor and it would only need to do something similar for the HI density.

Thanks,
Pedro.