(1) Updated the greenhouse gas accounting method for CO2 and CH4. This way we can show results in both individual gas units and CO2-equivalent units. (2) Introduced a module for N2O. (3) Updated the Global Warming Potential (GWP) conversions in the emission factors. (4) Included a tutorial for how to use the new features with example solutions Improved Rice and Landfill Methane.

- Kristina Colbert

Fixed the undefined self.n2o reference. It was out of order and needed to be placed above the CO2 functionality for the improved rice solution. And N2O is not a gas used in the landfill solution, so I removed all references to N2O there.

(1) Added jupyter notebook tutorials for how to run the new greenhouse gas accounting features and climate modeling.
(2) Included a few features into the Start_Here jupyter notebook for new functionality.
(3) Made a test notebook for the thirteen multi-gas solutions.
(4) Changes to the init.py files for the new ghg accounting functions.