Add access to Fermi example data file
Move examples to docs folder
update estimate_significance_image.py example
Move estimate_significance_image.py example to background docs
Add `utils.coordinates.spherical`

Move some functions from `utils.coordinates.celestial` there.
Add utils.coordinates.spherical.pixel_solid_angle function.

(completely untested)
Add image.utils.solid_angle function

(completely untested)
Add missing license line in utils/distributions files
Continue pixel solid angle implementation; add first tests