Refactor Domain UserObject

Model extents nx, ny and nz are now stored in the model. Currently these
are copied to FDTDGrid, but will be changed to local grid extents when
MPI implementation is added

gprMax/model.py

@@ -56,6 +56,10 @@ class Model:
     def __init__(self):
         self.title = ""
 
+        self.nx = 0
+        self.ny = 0
+        self.nz = 0
+
         self.G = self._create_grid()
         # Monitor memory usage
         self.p = None
@@ -127,6 +131,10 @@ class Model:
 
     def build_geometry(self):
         logger.info(config.get_model_config().inputfilestr)
+        # TODO: Make this correctly set local nx, ny and nz when using MPI
+        self.G.nx = self.nx
+        self.G.ny = self.ny
+        self.G.nz = self.nz
         self.G.build()
 
     def reuse_geometry(self):