JGCRI/gcam-core: GCAM 6.0

New features include:

1. A new residential floorspace expansion model.
2. Bio-energy updates: additional limits to "unsustainable" deployment
3. Default hotelling rate for climate stabilization scenarios is now 3%.
4. Split out 6 detailed industrial sectors from the aggregate industry sector.
5. Updated Hydrogen production, distribution, and end-use technologies.
6. A new protected lands definition.
7. Expanded crop commodities.
8. Use spatially explicit soil and vegetation carbon data from Moirai.
9. HFC MAC curve fixes.
10. New pollutant emissions controls.
11. Add direct air capture as a mitigation option in GCAM-USA.
12. Gross electricity trade in GCAM-USA.
13. Add GHGs to GCAM-USA.
14. Add pollutant emissions to GCAM-USA.
15. Solution improvements, particularly related to water markets.
16. Change the XML parser library to RapidXML.
17. Add the ability to exit the model early due to solution failure.
18. Reduce memory usage (offsets additional memory from expanded crop commodities).
19. gcamdata: Renv and user modification chunks.