Do not modify product list in processing plugins

[pnuu]

Feature Request

Supersedes #28, which I think is caused by this.

Is your feature request related to a problem? Please describe.
Modifying the product list in the processing step (e.g. adding the filenames to formats section` is a source for some problems:

1. it is not possible to define different formats settings for different areas, at it causes a crash with TypeError: save_dataset() got multiple values for keyword argument 'filename'
2. For developer, it is unclear what's the contents of the product list at given processing step, and one can't rely to get the same values which are provided in the configuration file

Describe the solution you'd like
The product list should stay unmodified all the way through the processing steps. All the new information created in each plugin should be added as a new entry in the job dictionary. And each plugin should make sure to check that the location they are adding their own stuff is empty before adding anything to it.

Describe any changes to existing user workflow
No changes, apart from making it possible to define different formats for different areas and simplifying the structure of job for developers.

Additional context
Have you considered any alternative solutions or is there anything else that would help describe your request.