update xattree version, update xatspec usage

Author: wpbonelli

flopy4/mf6/adapters.py

@@ -185,7 +185,7 @@ def __init__(
             self._data = package.data
         else:
             raise Exception("Input package has no data")
-        self._spec = get_xatspec(type(package))
+        self._spec = get_xatspec(type(package)).flat
         if modelgrid:
             self._grid = modelgrid
         elif model:

flopy4/mf6/codec/converter.py

@@ -29,7 +29,7 @@ def structure_array(value, self_, field) -> NDArray:
 
     # get spec
     spec = get_xatspec(type(self_))
-    field = spec[field.name]
+    field = spec.flat[field.name]
     if not field.dims:
         raise ValueError(f"Field {field} missing dims")