Fix silx deprecation warning

The function silx.io.dictdump.dicttoh5 now issues the following warning:

WARNING:silx.DEPRECATION:argument overwrite_data is deprecated since silx version 0.15. Reason: `overwrite_data=True` becomes `update_mode='modify'` and `overwrite_data=False` becomes `update_mode='add'`. Use 'update_mode' instead.
  File "/scisoft/users/paleo/.venv/py37/lib/python3.7/site-packages/silx/io/dictdump.py", line 257, in dicttoh5
    since_version="0.15",