Fix `coordinate_iau_format` doctest
Add `gammapy/data/tev_spectra/*` to `gammapy.data.get_package_data`.
Remove `aperphot` and `measure_bounding_box` functions.

This functionality is available in `photutils` and `scikit-image`.
Skip `healpix_to_image` doctest
Change `fermi_galacti_center` function to `FermiGalacticCenter` class
Add unit tests for FermiGalacticCenter
Improve sphinx formatting in install.rst
Fix doctest in introduction.rst
Add doctest skip directive to fit_sherpa.py (doesn't work)
Fix FermiGalacticCenter data example (use static methods)
Fix image.utils.coordinates doctest
Fix gaussian_sum_moments doctest
Skip doctest for MorphModelImageCreator
Skip doctests in gammapy.stats.utils
Fix doctest in gammapy.utils.const.conversion_factor
Skip doctest for gammapy.utils.root.TH2_to_FITS
Skip doctest in sherpa-chi2asym.py by reformatting the docstring
Docstring fixes and skip all doctests in gammapy/stats/poisson.py