Update to Python 3.9 and latest `xarray`

This release includes:

• upgrade to Python 3.9 and Xarray 2.3
• no more problem of memory byt using open_mfdataset
• new testing script with github action
• added units to default downscaled dataset
• n_core option is less restrictive now.
• solved a number of path issues
• a number of improvements of the FSM simulation topo_sim.py routines
• corrected projection issues when interpolating the climate dataset
• to_cryogrid() now compatible with Cryogrid.jl
• added an environment.yml file for version controlled install of VE
• plotting function map_variable() has been generalized
• and a number of bugs have been caught along the way.