Station plots

Starting to add some plots to Station object classes..

 - [x] Response: Bode plot
 - [ ] Channel: Bode plot (take Nyquist as upper bound)
 - [ ] Station: maybe with a kwarg switch (e.g. `type=..` one of `"response", "location", "both"`)
      - plot station location on global map
      - Bode plot with all channels (either all channels in one set of plots *or* subplots for each channel *or* separate figures for each channel)
 - [ ] Network/Inventory: Location plot like with Catalog

I think we can include this as additional minor features in the 0.9.x branch.. any objections?

Comments?