A new script generate_scenarios.py generates a separate config.yaml
file for each sector_opts string, so that all settings are
transparently visible in the config.yaml.

The script prepare_sector_network.py translates settings from the
config.yaml into a network object.

This allows easier identification of changes in parameters
sweeps. This will allow listing of parameter changes in a graphical
interface to interogate results (like PyPSA-server).

Hopefully it also lowers the chances of bugs in translating string
scenario names into changes.