Series.resample: fix numpy warning
TimeSeries.crop: rewritten for optimisation
StateVector: improved instance creation
StateVectorDict: simple subclass of TimeSeriesDict

    - mainly for frame reading
StateVector.resample: resample by logical AND
io: implement multi-processed resampling

    - resample each frame as it comes in to speed up join
    - need to give the right class from the off to catch
      subclass-specific resampling methods (StateVector)