Add HDF5 saving and concatenating to geco_gwpy_dump

[stefco]

• actually check the jobspec for the desired extension
• save downloaded timeseries to hdf5 files
• to concatenate files, load them again
• for each timeseries t, run t.copy() to get a new copy that can be worked with
• make a dict full of timeseries to append to from the first timeseries loaded
  (make sure to also t.copy() this one)
• append the copied timeseries in order to the timeseries dict
• save the final timeseries dict