add C increment variables to ForC

[teixeirak]

These would be very valuable for C accounting-- far more so than fluxes.

[teixeirak]

I've started adding these; still need to:

• [ ] add equations
• [x] reconcile NEP with delta.total.ecosystem_C
• [x] rename delta.agb?
• [x] add to IPCC variable mapping spreadsheet

[teixeirak]

This is essentially done. I don't think filling in equations is essential right now, so closing this.