add Bode plot to Response object
changelog
Response plot: add line cross for instrument sensitivity
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
Response/Channel plot: make it possible to pass in label string for
legend
Station plot: response plot working but might need some more work..
Station.plot(): only leave option to plot response

makes it simpler and clearer, for a map location plot it is probably
easier to nest the station into a network and use that for plotting
Station.plot(): take location codes into account
Merge remote-tracking branch 'upstream/master' into station_plots_master
station class plots: add example plots to docs
station type plots: fix plotting grid (just `grid()` toggles grid state)
Station.plot(): remove start/end stage options...

makes no sense in many cases because responses for channels might be
pretty different and matching a range of stages across channels
usually won't make much sense.
Response.plot(): autodetect sampling rate of channel from response stages
Response.plot() style changes and refactoring
bode plot: correct flipped phase
bode plot refactoring
bode plot: make phase unwrapping optional, default False
Merge remote-tracking branch 'upstream/master' into station_plots_master
inventory/network: response plots