Adds (partially) support to read only extract of particle data.
Turned plot_tuneshift into class TuneshiftPlots with more drawing options.
Turns plot_thresholds into a class ThresholdPlots.
Stores epsn_abs data in dictionary.
Adds (full) support to read in only part of the data.
Adds support to read in particle data using list of keys.
Adds calculate_spectrogram(analyzer). Method to be called by the user.

  - Before it was either calculate_fft_spectrogram or calculate_sussix_spectrogram.
  - These are now 'private' methods of the class.
Minor cleaning of method load_particle_data().
Minor cleaning of class and finalization.
Renames arguments of read_particle_data method to more meaningful names.
Adds new class to generate tune footprints (and their evolution) more easily.
Adds example code to generate tuneshift, threshold and/or footprint plots.

  - Includes some standard examples of output.
  - Input data are not provided.