E3SM-Project/e3sm_diags: v2.0.0

Some major changes in this new release:

• Refactored the code structure to make the code base more flexible to incorporate new diagnostics sets.
• Introduced a new way of running: run with API (i.e. python run_e3sm_diags.py). The old way (e3sm_diags -p) is backward compatible for the core diagnostics sets.
• Added time-series diagnostics for annual mean trend for selected variables (can be ran only with the new way)
• Variables P-E from (GPCP and OAflux), and from Core v2 fluxes are added.
• Dataset class is refactored to be cleaner and more flexible.
• Documentation is updated to reflect the new changes.