HDF5 saving for sparse data

After data reduction (on ID29), data are stored as sparse data, following the schema suggest by J. Kieffer:

$ h5ls -r sparse.h5
/                        Group
/entry_0000              Group
/entry_0000/beamline     Group
/entry_0000/beamline/Eiger\ 1M Group
/entry_0000/beamline/Eiger\ 1M/beam_center_x Dataset {SCALAR}
/entry_0000/beamline/Eiger\ 1M/beam_center_y Dataset {SCALAR}
/entry_0000/beamline/Eiger\ 1M/distance Dataset {SCALAR}
/entry_0000/beamline/Eiger\ 1M/x_pixel_size Dataset {SCALAR}
/entry_0000/beamline/Eiger\ 1M/y_pixel_size Dataset {SCALAR}
/entry_0000/beamline/sparse_frames Group
/entry_0000/beamline/sparse_frames/background_avg Dataset {1800, 80}
/entry_0000/beamline/sparse_frames/background_std Dataset {1800, 80}
/entry_0000/beamline/sparse_frames/dummy Dataset {SCALAR}
/entry_0000/beamline/sparse_frames/errors Dataset, same as /entry_0000/beamline/sparse_frames/background_std
/entry_0000/beamline/sparse_frames/frame_ptr Dataset {1801}
/entry_0000/beamline/sparse_frames/index Dataset {10311750}
/entry_0000/beamline/sparse_frames/intensity Dataset {10311750}
/entry_0000/beamline/sparse_frames/mask Dataset {1065, 1030}
/entry_0000/beamline/sparse_frames/radius Dataset {80}
/entry_0000/beamline/sparse_frames/script Dataset {SCALAR}
/entry_0000/beamline/sparsify Group
/entry_0000/beamline/sparsify/configuration Group
/entry_0000/beamline/sparsify/configuration/data Dataset {SCALAR}
/entry_0000/beamline/sparsify/configuration/type Dataset {SCALAR}
/entry_0000/beamline/sparsify/date Dataset {SCALAR}
/entry_0000/beamline/sparsify/program Dataset {SCALAR}
/entry_0000/beamline/sparsify/sequence_index Dataset {SCALAR}
/entry_0000/beamline/sparsify/version Dataset {SCALAR}
/entry_0000/end_time     Dataset {SCALAR}
/entry_0000/program_name Dataset {SCALAR}
/entry_0000/start_time   Dataset {SCALAR}