xarray fix and update to documentation
xarray 2023.9.0 introduced a change that broke FaIR with python 3.9+. Following user feedback, documentation has been updated, including, most importantly, an example using the latest available calibration. The positive semi-definite RuntimeWarnings should have also disappeared when running stochastically, or at least can be disabled.
Support for python 3.6 is removed due to dependency issues, and 3.11 added.