SegmentAxes: latch inset labels to xlim[0]

    - new set_xlim calling super() and then resetting the
      x-position of all inset segment labels
TimeSeries.read: use multiprocessing

    - experimental feature for separating cache into N chunks, reading
      each on in its own process, then join()ing
    - seems to work fine on ldas-pcdev2.ligo-la.caltech.edu via gwsumm