verify list of valid_fields and add comments for generated fields

[szabgab]

In the cat/code.py there is now a list of valid fields for the event files.
Check that those are really the fields we would like to accept and add some comments to the ones that are generated by the code. Better yet, if possible, move the code that validates the fields to an earlier place where there are no generated fields yet and remove the generated fields from the list of valid_fields.