data: move custom units into data.__init__:

    - all Array sub-classes will pick up custom units
Array.{from,to}_hdf5: added HDF methods

    - custom array writing in HDF5 format
    - allows unified I/O for each subclass
timeseries.io.losc: remove auto-id and 'hdf' spec
timeseries.io.hdf: added HDF5 unified I/O
Spectrogram: fix init for channel with no sampling
spectrogram.io: added HDF I/O
spectrum.io.hdf5: added HDF5 I/O