fix imports in mu map dataset

mu_map/data/datasets.py

 import os
 from typing import Optional
 
+import cv2 as cv
 import pandas as pd
 import pydicom
 import numpy as np
@@ -129,8 +130,6 @@ __all__ = [MuMapDataset.__name__]
 if __name__ == "__main__":
     import argparse
 
-    import cv2 as cv
-
     from mu_map.util import to_grayscale, COLOR_WHITE
 
     parser = argparse.ArgumentParser(