Add CityCatalyst

[Ly0n]

Insert URLs to the project here:
https://github.com/Open-Earth-Foundation/CityCatalyst

In one sentence, explain what the project is about:
Open Source carbon accounting for cities

• [x] The projects is active, documented, open source licensed, shows usage from external parties and is directly targeting environmental sustainability. Find more details in the Contribution Guide.

Documentation: https://github.com/Open-Earth-Foundation/CityCatalyst/wiki