UI to toggle pre existing states

[RELNO]

This feature will allow loading of preexisting state via UI.

• Users will select a scenario from a UI
• The scenario will load with it's pre-computed layers
• The scenario could be used as template for designing a future design, sort of a starting point
• Scenarios will use the current API of URL query to load, so that not all CS endpoints will hold pre existing states
• The loading mechanism should follow current cityIO reading as much as possible (no special API should be introduced), only that data should be local

[RELNO]

closed with #75