Move iterations and timewindow to FDTDGrid

Grids need to know the number of iterations and the timewindow to
calculate waveform values. While waveforms are global in an MPI sense,
subgrids will calculate their own waveforms based on the dt value of the
subgrid.

gprMax/grid/fdtd_grid.py

@@ -61,6 +61,9 @@ class FDTDGrid:
         self.dl = np.ones(3, dtype=np.float64)
         self.dt = 0.0
 
+        self.iterations = 0  # Total number of iterations
+        self.timewindow = 0.0
+
         # Field Arrays
         self.Ex: npt.NDArray[np.float32]
         self.Ey: npt.NDArray[np.float32]

gprMax/model.py

@@ -56,8 +56,6 @@ class Model:
         self.dt_mod = 1.0  # Time step stability factor
 
         self.iteration = 0  # Current iteration number
-        self.iterations = 0  # Total number of iterations
-        self.timewindow = 0.0
 
         self.srcsteps = np.zeros(3, dtype=np.int32)
         self.rxsteps = np.zeros(3, dtype=np.int32)
@@ -141,6 +139,22 @@ class Model:
     def dt(self, value: float):
         self.G.dt = value
 
+    @property
+    def iterations(self) -> int:
+        return self.G.iterations
+
+    @iterations.setter
+    def iterations(self, value: int):
+        self.G.iterations = value
+
+    @property
+    def timewindow(self) -> float:
+        return self.G.timewindow
+
+    @timewindow.setter
+    def timewindow(self, value: float):
+        self.G.timewindow = value
+
     def _create_grid(self) -> FDTDGrid:
         """Create grid object according to solver.