Add direct emissions method with data export

[kndehaan]

Context

This PR adds a new direct emissions method to the model, together with a detailed data export containing all greenhouse gas emissions of a scenario.

Implemented changes

• New engine methods and node attributes that calculate relevant direct emissions, based on presence of (molecule) node group emissions
• Data processing pipeline that generates start year emissions data for country datasets, based on reported emission data from UNFCCC. Start year data is added as emissions.csv file to ETSource using a rake task.
• New molecule nodes that read static emission data from emissions.csv, specified for co2 and other_ghg
• Detailed data export containing relevant direct reporting emission results for all relevant nodes, and categorises these into ETM sector and subsector.

Related

Goes with pull requests:

• Emissions etengine#1754
• Emissions atlas#189
• Add direct emissions method with data export etsource#3444
• Add direct emissions method with data export #4719
• https://github.com/quintel/etdataset/pull/1087

Checklist

• I have tested these changes
• I have updated documentation as needed
• I have tagged the relevant people for review