response plot: changes to call parameters

- more intuitive for users to be able to specify sampling rate +
  frequency step (i.e. min/max/step of frequencies) in plot (rather than
  t_samp and nfft)
- make it able to provide two axes for the plotting instead of making a
  new figure (will be used in plotting multiple channels in
  Station/Network/Inventory)
response plot for Channel class