DataMonitor: added labelling
SpectrumMonitor: fix for new buffer type

    - added filtering
TimeSeriesMonitor: minor change in update_data
DataMonitor: minor change

    - should have no effect under normal circumstances
BufferCore: minor bug fix
BufferCode: append data in place by default
NDSDataIterator: two minor changes
SpectrumMonitor.update_data: always generate spectrum

    - if elapsed time is less than fftlength, shorten fftlength to
      calculate something
    - always use the end of the data buffer (most recent data) rather
      than the TimeSeries.asd default of the using the start