Use posixpath to construct HDF5 paths

src/cal_tools/agipdlib.py

 import traceback
 import zlib
 from multiprocessing.pool import ThreadPool
+import posixpath
 from pathlib import Path
 from typing import Any, Dict, Optional, Tuple
 
@@ -966,11 +967,11 @@ class AgipdCorrections:
         # these are touched in the correct function, do not copy them here
         dont_copy = ["data", "cellId", "trainId", "pulseId", "status",
                      "length"]
-        dont_copy = [agipd_base.rstrip('/') + f"/image/{ds}"
+        dont_copy = [posixpath.join(agipd_base, "image", ds)
                      for ds in dont_copy]
 
         # don't copy index as we may need to adjust if we filter trains
-        dont_copy.append(idx_base.rstrip('/') + "image")
+        dont_copy.append(posixpath.join(idx_base, "image"))
 
         h5_copy_except_paths(infile, outfile, dont_copy)