Prepare Pyxel for its open-source releasing

Before releasing Pyxel as an open-source project via the European Space Software Repository (ESSR) (see https://essr.esa.int), the following steps have to be fulfilled:

• Use the ESA Public License (ESA-PL) Permissive - v2.3 (see https://essr.esa.int/license/european-space-agency-public-license-v2-3-permissive).
• Add ESA copyright in each files.
• Remove FITS files that are not property of ESA (in folder pyxel/data/fits)

More relevant links:

• ESA Open Source Policy: https://essr.esa.int/esa-open-source-policy
• ESA Public license commentary - v2.3: https://essr.esa.int/license/european-space-agency-public-license-esa-pl-commentary-v2-3

Subjects open to discussion:

• Do we have to add a copyright in the YAML files ? (pyxel\examples\*.yaml)
• What do to with these files :
  • pyxel/models/charge_collection/persistence.py
  • pyxel/models/charge_measurement/nghxrg/nghxrg_beta.py
  • pyxel/_version.py
  • pyxel/versioneer.py
  • pyxel/setup.py
  • pyxel/tests/**/test_*.py