Several fixes

Signed-off-by: emanuele <emanuele45@gmail.com>
Added  symfony/yaml as dependency via composer

Signed-off-by: emanuele <emanuele45@gmail.com>
Convert importer skeleton to the yaml format and add a basic function to ensure the expected values are all present

Signed-off-by: emanuele <emanuele45@gmail.com>
Added support for default values in the importer_skeleton.yml file

Signed-off-by: emanuele <emanuele45@gmail.com>
Started fixing Elk attachments import and removed debug code

Signed-off-by: emanuele <emanuele45@gmail.com>