Revert "Refactor AddGress UserObject build process"

This reverts commit 1d7b5ff0f9.

gprMax/model.py

@@ -26,7 +26,6 @@ import numpy as np
 import psutil
 from colorama import Fore, Style, init
 
-from gprMax.fractals import FractalVolume
 from gprMax.grid.cuda_grid import CUDAGrid
 from gprMax.grid.opencl_grid import OpenCLGrid
 from gprMax.materials import ListMaterial, Material, PeplinskiSoil, RangeMaterial
@@ -68,7 +67,6 @@ class Model:
         self.subgrids: List[SubGridBaseGrid] = []
 
         self.mixingmodels: List[Union[PeplinskiSoil, RangeMaterial, ListMaterial]] = []
-        self.fractalvolumes: List[FractalVolume] = []
 
         self.geometryviews: List[GeometryView] = []
         self.geometryobjects: List[GeometryObjects] = []