spectoh5: attributes not created when creating groups or datasets

[PiRK]

This is a bug in my code.

When reading a SpecFile and interpreting it as a HDF5 with silx.io.spech5, HDF5-like attributes are attached to some groups and datasets as a dictionary. For instance, the root group has the following .attrs: {"NX_class": "NXroot"}.

These attributes are currently not being written to the output HDF5 file when converting a SpecFile using functions from silx.io.spectoh5.