Fix materials only built when have dispersive materials

gprMax/grid/fdtd_grid.py

@@ -81,6 +81,11 @@ class FDTDGrid:
         self.rigidH: np.ndarray[Any, np.dtype[np.int8]]
         self.ID: np.ndarray[Any, np.dtype[np.uint32]]
 
+        # Update Coefficient Arrays
+        self.updatecoeffsE: np.ndarray
+        self.updatecoeffsH: np.ndarray
+        self.updatecoeffsdispersive: np.ndarray
+
         # PML parameters - set some defaults to use if not user provided
         self.pmls = {}
         self.pmls["formulation"] = "HORIPML"