Config settings not on the same level as scenario configurations

[jnnr]

Some settings are defined centrally in config.settings, others are defined scenario-wise in the yaml files in scenarios/. It would be desirable to put both configs on the same footing. This would make it easy to move those settings that happen to be the same in all scenarios to the settings. The scenarios would just update the settings.

I am not sure yet how this can be done. Currently, only the script build_datapackage uses the scenario information. It loads the scenario specs only in the lower part of the script, but the config is imported at the very begging and used in the functions.

What do you think, @henhuy?