Added CO2 analysis and general code clean up

[chantalwuer]

• added class_allocation_CO2.py to calculate allocations for CO2 only
• refactored class_allocation.py to improve readability (used the new version as a template for the CO2 analysis)
• started a folder with testing scripts; added first script to test country allocation files

The CO2 allocation is the first version and still needs a second pair of eyes to double check. Mostly straightforward except for PCB.