Station improvements (based on master)

Starting to add some plots to Station object classes..

 - [x] Response: Bode plot
 - [x] Channel: Bode plot (take Nyquist as upper bound)
 - [x] 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)
 - [x] Network/Inventory: Bode plot
 - [x] Network/Inventory: Location plot like with Catalog
 - [x] Add image tests
 - [x] Add example plots / images in API docs
 - [ ] bode plot: only annotate sensitivity if the chosen output units match the overall sensitivity units?
 - [ ] annotate correct amplification units (e.g. "count/ m/s"; could be parsed from start/end stage but has to take chosen output units into account)
 - [ ] wrap bode plots in a class in imaging (to avoid code duplication, enhance custom reusability, ..)?
 - [x] use time stamp to restrict matching channels
 - [x] include plots in gallery
 - [ ] include option to plot PAZ-stage-only as alternative for comparison
 - [ ] tests for new methods added to station objects (select(), is_active())
 - [ ] `__len__` attribute for Inventory/Network/Station?
 - [ ] `Inventory.count(level="network", count_empty=True)` method?

Comments?