Interpretation of total debt & government debt variables

[danielhuppmann]

In #279, @FlorianLeblancDr added the variables "Debt" and "Debt|Government". @chrisdwells asked (by email) whether this is a yearly flow (increase in debt?) or the total debt in that year?

@IAMconsortium/common-definitions-coordination @IAMconsortium/common-definitions-macro-economy