Add depth property in uxgrid

This provides consistency with the XGrid class. Additionally,
this property is needed for determining min/max depth bounds in
particleset.__init__ . With consistency between XGrid and UXGrid, this
getting depth ranges with either grid type can be accomplished with the
same code in particleset.__init__

Author: fluidnumericsJoe

parcels/uxgrid.py

@@ -35,6 +35,20 @@ def __init__(self, grid: ux.grid.Grid, z: ux.UxDataArray) -> UxGrid:
             raise ValueError("z must be a 1D array of vertical coordinates")
         self.z = z
 
+    @property
+    def depth(self):
+        """
+        Note
+        ----
+        Included for compatibility with v3 codebase. May be removed in future.
+        TODO v4: Evaluate
+        """
+        try:
+            _ = self.z.values
+        except KeyError:
+            return np.zeros(1)
+        return self.z.values
+
     def search(
         self, z: float, y: float, x: float, ei: int | None = None, search2D: bool = False
     ) -> tuple[np.ndarray, int]: