New variables to report land cover change transitions

[tgasser]

This is the second of two PRs to initiate a change in the AFOLU-related reporting, following a community-wide consultation carried out within the NewPathways HEU project.

It introduces land cover change transition variables that teams could report and that would enable a much better reanalysis of land use change emissions and by bookkeeping models such as OSCAR (see Gidden et al.).

Note that this is a first attempt at something not too overwhelming. Level-1 transitions could be removed for simplicity. This could also be written differently (and more easily) with some additional parsing rules for tags (like exceptions).

In addition, the nested structure of IAMC variables is ill-suited for multi-dimensional variables such as these transitions, which is also visible in the case of crops in land-cover as there's no clear priority in the variable tree between rainfed/irrigated and {crop-types} -- these require a horizontal structure instead of a vertical one.

Suggest @flohump has a look.

Resulting definitions file attached: definitions_lcc_matrix.xlsx.

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File structure changes:

• new variable file land-cover-change-matrix under afolu
• several new tag files with land cover types

Other changes:

• altered Land Cover|Other Natural subcategories to parallel that of forest with primary and secondary types and subtypes
• bumped up the tier of some land-cover variables