Export missing variables for trade and policy

[JulianGeis]

Add missing variables: for trade:

2 | energy (supply) | Trade\|Primary Energy\|Biomass\|Volume | xx | not modelled | PJ/yr | net exports of solid, unprocessed biomass, at the global level these should add up to the trade losses only -- | -- | -- | -- | -- | -- | -- 2 | energy (supply) | Trade\|Secondary Energy\|Electricity\|Volume | xx | added  | PJ/yr | net exports of electricity, at the global level these should add up to the trade losses only 2 | energy (supply) | Trade\|Secondary Energy\|Hydrogen\|Volume | xx |  added | PJ/yr | net exports of hydrogen, at the global level these should add up to the trade losses only 2 | energy (supply) | Trade\|Secondary Energy\|Liquids\|Hydrogen\|Volume | xx | not modelled? | PJ/yr | net exports of liquid hydrocarbon fuels from renewable hydrogen and renewable CO2 (green synfuels), at the global level these should add up to the trade losses only 2 | energy (supply) | Trade\|Secondary Energy\|Gases\|Hydrogen\|Volume | xx | can not be distinguished? | PJ/yr | net exports of gaseous hydrocarbon fuels from renewable hydrogen and renewable CO2 (green syngases), at the global level these should add up to the trade losses only Trade\|Primary Energy\|Coal\|Volume | x | not modelled? | PJ/yr | net exports of coal, at the global level these should add up to the trade losses only -- | -- | -- | -- | -- Trade\|Primary Energy\|Gas\|Volume | x | not modelled? | PJ/yr | net exports of natural gas, at the global level these should add up to the trade losses only Trade\|Primary Energy\|Oil\|Volume | x | not modelled? | PJ/yr | net exports of crude oil, at the global level these should add up to the trade losses only and policy: policy | Price\|Carbon\|ETS | x | same as global target? | EUR2020/t CO2 | Price of carbon within the EU ETS -- | -- | -- | -- | -- | -- policy | Price\|Carbon\|EU-wide Regulation Non-ETS | x | not modelled? | EUR2020/t CO2 | Price of carbon implied by EU-wide Non-ETS sector target policy | Price\|Carbon\|National Climate Target | x | same as national target? | EUR2020/t CO2 | Price of carbon due to national policies for all sectors - either from a direct carbon price, or as result of a national climate target policy | Price\|Carbon\|National Climate Target Non-ETS | x | not modelled? | EUR2020/t CO2 | Price of carbon due to national policies for the non-ETS sectors - either from a direct carbon price, or as result of a national climate target for these sectors Results:  

[lindnemi]

Looking good so far. Two more points:

• [x] Trade|Secondary Energy|Liquids|Hydrogen|Volume can be modeled with the DE renewable oil -> EU oil and vice versa links
• [x] Trade|Primary Energy|Gas|Volume can be modeled with cross-border gas links. I would addtionally multiply with the EU-wide fossil gas share since the descirption mentions "natural gas"

[JulianGeis]

[lindnemi]

Codewise this PR looks good now. However, i am still a bit worried about the large swings in trade volume in later years:

After merging main: