Disu as graph (modflowpy#58)

* Show that it's possible to store a CSR matrix in an XArray Dataset

The test checks that it can restore the graph after saving it in CSR format.
Also added a dense test that takes simple numpy arrays.

* Auto-format and organize imports via ruff on save

.vscode/settings.json

@@ -3,5 +3,12 @@
         "test"
     ],
     "python.testing.unittestEnabled": false,
-    "python.testing.pytestEnabled": true
+    "python.testing.pytestEnabled": true,
+    "[python]": {
+        "editor.formatOnSave": true,
+        "editor.defaultFormatter": "charliermarsh.ruff",
+        "editor.codeActionsOnSave": {
+            "source.fixAll": "explicit"
+        }
+    }
 }