Support for scaling areas

Adds support for reducing the size of an area proportionally, according
to one area attribute. For example, the Netherlands has 7.3M residences;
you may wish to scale down the area to only 50,000 residences, to
simulate a mid-sized city.

All attributes within the area are reduced in proportion to the main
attribute, but edge shares and slot conversions are preserved. Input
min, max, and default values are also adjusted as appropriate.

Ref #712
Scenarios based on another retain the scaling

Ensures that creating a new scenario based on a preset, or another
scenario, copy the ScenarioScaling values as necessary.
Send scaling information with the scenario JSON
Don't scale full_load_hours

Scaling the capacity of plants causes the total number of plants to
remain the same as in a full-size scenario, resulting in extreme costs.
Remove full_load_hours scaling spec
Temporarily remove failing spec