TimeSeries.append/prepend: allows non-zero pad
TimeSeriesList: new list sub-class

    - entries must be TimeSeries
    - methods to coalesce and join lists of TimeSeries
NDS: redesigned NDS interface

    - io.nds.NDS2Connection now a thin sub-class of nds2.connection
    - TimeSeries.fetch now handles all type-casting, allowing users to
      pass in open connections either nds2.connection or sub-class