Add xarray to test environment

.github/workflows/testing.yml

@@ -25,7 +25,7 @@ jobs:
         sudo apt-get update
         sudo apt-get install gfortran liblapack-pic liblapack-dev libnetcdf-dev libnetcdff-dev
         python -m pip install --upgrade pip
-        pip install flake8 meson-python ninja pytest numpy pandas
+        pip install flake8 meson-python ninja pytest numpy pandas xarray
         # if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
 
     - name: Checkout Clawpack