JGCRI/xanthos: Xathos v2.0.0

Xanthos v2.0.0 is now set as a framework where core components of the model (PET, runoff, and river routing) can be interchanged or extended without having to start from scratch allowing researchers to quickly integrate and test their new modules and configurations.

Major advancements for Xanthos 2.0 were also achieved by creating a more robust default configuration for the model that is now available to the scientific community. These advancements include the addition of: the Penman-Monteith PET module to capture the impacts of evolving land cover, the ABCD water balance module to account for groundwater recharge and discharge in runoff projections, improved water velocity considerations for the Modified River Transport Model (MRTM) routing module, a built-in differential evolution optimization module to calibrate ABCD parameters to modeled global runoff, and hydropower production assessment and potential capacity modules.