Refactor time series code, add tab widget for fft plot.
Implement DFT amplitude plot.

For the selected fields, and the selected time range of interest, the DFT is
computed and the amplitude is ploted. Lots of polishing on the amplitude plot
is needed:

  * selecting lines doesn't highlight them in the legend.
  * reset of axes only works on time series plot
  * you have to switch back to the time series plot if you want to add fields
    for which the DFT is computed.
  * COLORS NEED TO BE IMPLEMENTED!
  * data output needs to be implemented so FFT plots can be made.