Export import varaibles / based on NAVIGATE / ENGAGE

[FlorianLeblancDr]

This PR adds Export Import variables from NAVIGATE / ENGAGE

Some macro sectors are missing but will dynamically change by updating the tag (#279 )
Export|Energy
Export|Industry|Chemicals
Export|Industry|Manufacturing
Export|Industry|Energy
Export|Other
Export|Industry|Energy Intensive

Discarded (can be computed within the database):
Exports|% change per GDP
Export|Developing Country Share
Exports|% change from Ref

[FlorianLeblancDr]

Having second thoughts, I am moving export and import variables to the trade.yaml files
and improve clarity of the use of variable name with a header

[FlorianLeblancDr]

Looking at NAVIGATE and ENGAGE "Trade" variables, I further discarded:
"Trade|Emissions|Value|Carbon*" variables (redundant with "Trade|Emissions Allowances*"
"Trade|Gross Import|Secondary Energy|Electricity|*": we can add Export and Import of energy in a further release

[FlorianLeblancDr]

@volker-krey @macflo8
This a good example on implementing both ISIC sector nomenclature and biophysical nomenclature.
The same can be done for Production, for exampel.