Separate reporting of positive and negative carbon flux on land

[gidden]

In the @IAMconsortium/common-definitions-emissions task team, we are discussing how to account for emissions and removals from land-related processes.

We already explicitly have a variable for net LULUCF flux and carbon sequestration on land (which will be elaborated on). This implies that we should also be able to report a net positive carbon flux on land as well. We would propose to do so in similar land categories as are currently used elsewhere.

Thoughts and opinions welcome @IAMconsortium/common-definitions-land

cc @strefler / @jayfuhrman