Move image_to_hdu to pyfact.py
Temporarily add PyFACT stuff in gammpy/utils/pyfact.py
Add some entries to docs/references.rst
chmod scripts to be executable
Move over `sim_evlist` and `create_spectrum` from PyFACT
Add `EnergyDispersion.apply` method
Add `gp-pfmap` script (adapted from `pfmap` script from PyFACT)
Add `gp-pfsim` script (adapted from `pfsim` script from PyFACT)
Whitespace formatting cleanup