Speed-up and restructure plotting

[chipmuenk]

• Currently, ax.clear() is called before each plot, slowing down redraw considerably. This should only happen in init_axes() which needs to be called during initialization and when important plot settings (like the number of data points or subplots) have changed. Current status is
  self.ax.clear() # need to clear, doesn't overwrite
line_phi, = self.ax.plot(F, phi_plt)
  A better solution would be using
  set_ydata(...) together with restoring the background and fig.canvas.blit(...)
  See e.g. https://stackoverflow.com/questions/8955869/why-is-plotting-with-matplotlib-so-slow

• When only the view has changed (lin / log, units, min/max), use update_view()

• When the filter has been updated, use draw() calls calc_hf()

• Rename draw() to something like update_data()

• Rename calc_hf() to calc_plot()