Draft: Add NetCDF output format

This adds NetCDF as a new, optional EnPT output format.

Tasks to be done:

• provide NetCDF template to which we could adapt the EnPT output
• add NetCDF as a new option to the output format configuration parameter
  • adapt config files: config.py, options_default.json, options_schema.py
  • adapt command line argument parser(enpt_cli.py)
  • adapt GUI (enpt_enmapboxapp repository, here)
• adapt the EnPT L2A output writer to include the NetCDF file format there
• update documentation (should at least mention the option to use NetCDF but should also give a broad overview about the NetCDF-internal data structure)
• update HISTORY.rst