Fix default filename for features from GSTLAL_SNAX -> SNAX
See title. This should be more consistent with the filename convention downstream users would expect (e.g. https://gwpy.github.io/docs/stable/table/io/#snax-hdf5).
See title. This should be more consistent with the filename convention downstream users would expect (e.g. https://gwpy.github.io/docs/stable/table/io/#snax-hdf5).