Fix deprecation warning by making h5py file mode explicit
Fix the following deprecation warning:
H5pyDeprecationWarning: The default file mode will change to 'r'
(read-only) in h5py 3.0. To suppress this warning, pass the mode
you need to h5py.File(), or set the global default
h5.get_config().default_file_mode, or set the environment
variable H5PY_DEFAULT_READONLY=1. Available modes are: 'r', 'r+',
'w', 'w-'/'x', 'a'. See the docs for details.